Adds a new systemd unit to the image that will be pulled in by default,
run a given command, forward the output to a virtio serial port and
shutdown the machine.
We add a sample that uses this to verify that systemd conciders the
machine successfully booted. A simple way to run this test from the
commandline is to use
`$ socat UNIX-LISTEN:qemu.sock -`
to listen for either `running` for success or `degraded` or
`maintenance` for failure.
The image should then be booted using something like
`$ qemu-kvm -m 1024 -nographic -monitor none -serial none -chardev socket,path=qemu.sock,id=char0 -device virtio-serial -device virtserialport,chardev=char0,id=test0 -snapshot base.qcow2`
Signed-off-by: Tom Gundersen <teg@jklm.no>
