Reboot in the tutorial can be stuck at initramfs by missing `/dev/sda2`

How to reproduce:

1. prepare LXD
```
$ snap info lxd | egrep 'installed:|tracking:'
tracking: latest/stable
installed: 5.15-002fa0f (25112) 181MB -
```

2. Follow the tutorial - https://canonical-microceph.readthedocs-hosted.com/en/latest/tutorial/install/

Sometimes, some of the machines can be stuck at the initramfs phase after executing the reboot in the "Prepare VMs" step. That is because the kernel command line has `root=/dev/sda2` but an additional drive is added in the previous step in the tutorial, so `/dev/sda` might be the additional disk not the original OS drive. This could be more of an LXD side of issues but I'd start here since it can damage the first user story.

```
(initramfs) Gave up waiting for root file system device. Common problems:
- Boot args (cat /proc/cmdline)
- Check rootdelay= (did the system wait long enough?)
- Missing modules (cat /proc/modules; ls /dev)
ALERT! /dev/sda2 does not exist. Dropping to a shell!

BusyBox v1.30.1 (Ubuntu 1:1.30.1-7ubuntu3) built-in shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) Gave up waiting for root file system device. Common problems:
- Boot args (cat /proc/cmdline)
- Check rootdelay= (did the system wait long enough?)
- Missing modules (cat /proc/modules; ls /dev)
ALERT! /dev/sda2 does not exist. Dropping to a shell!

BusyBox v1.30.1 (Ubuntu 1:1.30.1-7ubuntu3) built-in shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-5.15.0-78-generic root=/dev/sda2 ro quiet splash console=tty1 console=ttyS0 vt.handoff=7
(initramfs) ls -l /dev/sd* /dev/disk/by-id/ /dev/disk/by-path/
brw------- 1 8, 18 /dev/sdb2
brw------- 1 8, 17 /dev/sdb1
brw------- 1 8, 16 /dev/sdb
brw------- 1 8, 0 /dev/sda

/dev/disk/by-path/:
lrwxrwxrwx 1 9 pci-0000:02:00.0-scsi-0:0:1:1 -> ../../sda
lrwxrwxrwx 1 10 pci-0000:02:00.0-scsi-0:0:0:1-part1 -> ../../sdb1
lrwxrwxrwx 1 10 pci-0000:02:00.0-scsi-0:0:0:1-part2 -> ../../sdb2
lrwxrwxrwx 1 9 pci-0000:02:00.0-scsi-0:0:0:1 -> ../../sdb

/dev/disk/by-id/:
lrwxrwxrwx 1 9 scsi-0QEMU_QEMU_HARDDISK_lxd_osd--3 -> ../../sda
lrwxrwxrwx 1 10 scsi-0QEMU_QEMU_HARDDISK_lxd_root-part2 -> ../../sdb2
lrwxrwxrwx 1 10 scsi-0QEMU_QEMU_HARDDISK_lxd_root-part1 -> ../../sdb1
lrwxrwxrwx 1 9 scsi-0QEMU_QEMU_HARDDISK_lxd_root -> ../../sdb
```