CI: Add QEMU boot tests

CI: Add QEMU boot tests

Adding these gives us a clear idea of which setups should work and which
are broken. This helps with triaging issues since we can immediately see
if a setup is supposed to work or not. It also makes sure setups that do
work keep working in the future.

Currently, only Arch, CentOS and Fedora can boot on all included setups
(nspawn, UEFI + systemd-boot with unified kernel images, UEFI +
systemd-boot and BIOS). Debian, Ubuntu, OpenSUSE and Mageia fail
on UEFI without unified kernel images because their package managers
don't install any boot loader entries that systemd-boot can read.

Photon doesn't work on any of the setups. Clear also has a lot of issues
due to it using its own bootloader and OpenMandriva has QEMU failures.
For now, we disable all these but we hope support by them can be added
by interested contributors in the future.