[Bug 2121215] [NEW] arm64 and arm64+largemem questing installs fail with VFS: Unable to mount root fs on unknown-block(0, 0)

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Logs attached. For over a week now, the arm64 and arm64+largemem ISOs
fail to install, and aren't getting promoted. I'm not entirely sure of
the root cause, I've struggled to dig further. Here's the VM xml:

```
<domain type="kvm">
  <name>utah-6281-questing-live-server-arm64+largemem</name>
  <uuid>7de8d542-662d-46bb-aa9e-bee16ed84e8a</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <cpu mode="host-passthrough" />
  <os>
    <type arch="aarch64" machine="virt">hvm</type>
    <loader readonly="yes" type="pflash">/usr/share/AAVMF/AAVMF_CODE.fd</loader>
    <nvram template="/usr/share/AAVMF/AAVMF_VARS.fd">/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/nvram.fd</nvram>
    <boot dev="cdrom" />
  </os>
  <features>
    <acpi />
  </features>
  <clock offset="utc" />
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>destroy</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <controller type="scsi" index="0" model="virtio-scsi" />
    <disk type="file" device="cdrom">
      <driver name="qemu" type="raw" />
      <target dev="sdc" bus="scsi" />
      <readonly />
    <source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/tmp14yaow03/questing-live-server-arm64+largemem.iso" /></disk>
    <interface type="network">
      <mac address="52:54:00:2b:6e:ca" />
      <source network="default" />
      <model type="virtio" />
    </interface>
  <disk type="file" device="disk"><driver name="qemu" type="qcow2" cache="unsafe" /><serial>disk-0</serial><source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/disk0.qcow2" /><target dev="vda" bus="virtio" /></disk><disk type="file" device="disk"><driver name="qemu" type="raw" cache="unsafe" /><serial>disk-1</serial><source file="/var/lib/utah/vm/utah-6281-questing-live-server-arm64+largemem/tmp14yaow03/answers.img" /><target dev="vdb" bus="virtio" /></disk><serial type="file"><source path="/jenkins/servers/platformqa/workspace/ubuntu-questing-live-server-arm64+largemem-smoke-default/ARCHVARIANT/default/NODE/iso-testing-arm64+largemem/log/utah-6281.syslog.log" /><target port="0" /></serial></devices>
</domain>
```

And the relevant dmesg snippet:

```
[    1.671218] clk: Disabling unused clocks
[    1.672280] PM: genpd: Disabling unused power domains
[    1.673612] md: Waiting for all devices to be available before autodetect
[    1.673701] md: If you don't use raid, use raid=noautodetect
[    1.676462] md: Autodetecting RAID arrays.
[    1.677474] md: autorun ...
[    1.677540] md: ... autorun DONE.
[    1.679015] Failed to create /dev/root: -28
[    1.680198] /dev/root: Can't lookup blockdev
[    1.681294] VFS: Cannot open root device "" or unknown-block(0,0): error -2
[    1.681364] Please append a correct "root=" boot option; here are the available partitions:
[    1.681431] fd00        16777216 vda 
[    1.681506]  driver: virtio_blk
[    1.681565] fd10            1024 vdb 
[    1.681614]  driver: virtio_blk
[    1.681670] 0b00         2214300 sr0 
[    1.681717]  driver: sr
[    1.681772] List of all bdev filesystems:
[    1.681822]  ext3
[    1.681868]  ext2
[    1.681918]  ext4
[    1.681968]  squashfs
[    1.682017]  vfat
[    1.682067]  fuseblk
[    1.682171] 
[    1.682230] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[    1.694047] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.15.0-4-generic-64k #4-Ubuntu PREEMPT(voluntary) 
[    1.695968] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[    1.697279] Call trace:
[    1.697762]  show_stack+0x38/0xa0 (C)
[    1.698473]  dump_stack_lvl+0x84/0xc0
[    1.699179]  dump_stack+0x1c/0x40
[    1.699818]  panic+0x1a8/0x3e0
[    1.700409]  mount_root_generic+0x204/0x2e8
[    1.701214]  mount_root+0xa8/0xd8
[    1.701861]  prepare_namespace+0x1e0/0x290
[    1.702648]  kernel_init_freeable+0x1a4/0x1e0
[    1.703481]  kernel_init+0x3c/0x190
[    1.704158]  ret_from_fork+0x10/0x20
[    1.704898] Kernel Offset: disabled
[    1.705590] CPU features: 0x0000,00000268,01000400,0200021b
[    1.706663] Memory Limit: none
[    1.707258] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---
```

** Affects: livecd-rootfs (Ubuntu)
     Importance: Undecided
         Status: New

-- 
arm64 and arm64+largemem questing installs fail with VFS: Unable to mount root fs on unknown-block(0,0)
https://bugs.launchpad.net/bugs/2121215
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