Improve install device selection in installer

[yfyf]

• Look for "/iso" mount not only in children of blockdevices, but top-level devices too (e.g. this happens when running qemu with -cdrom playos-installer-<..>.iso)
• Eliminate devices that are too small or read-only from potential installation targets.

cc @knuton

Checklist

• Changelog updated
• Code documented
• User manual updated

[yfyf]

P.S. I was "inspired" to do this PR because I was running qemu with -cdrom playos-installer-XX.iso and it would fail to identify the mounted ISO as the boot medium (and hence would not filter it out from potential installation targets).

[knuton]

Looks good! A few minor suggestions.

[yfyf]

Resolved all the comments, tested manually by adding additional disks and checking that it picks the right one based on size:

Found 2 disk devices:
        /dev/vdb (Virtio Block Device - 1 GB)
        /dev/vda (Virtio Block Device - 28 GB)
Could not identify installer medium. Considering all disks as installation targets.
Minimum required size for installation target: 26 GB
Found 1 possible installation targets:
        /dev/vda (Virtio Block Device - 28 GB)

(Note: Installer medium is not identified because I am running via nix runInLinuxVM.)

[knuton]

tested manually by adding additional disks and checking that it picks the right one based on size

Did the following additional sanity checks on physical machine:

• The 32 GB installation medium is ignored
• Additional 8 GB device is ignored
• 120 GB disk is selected

[knuton]

Thanks 👌