Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
Direct kernel boot results in much faster boot times so let's use
it by default.
We disable it for tests that need to reboot because +-50% of the
time, doing a reboot when using direct kernel boot causes qemu to
hang on reboot. Until we figure that out, let's use UEFI for the
tests that need to reboot.
