automation: wait for the login prompt as test end marker

automation: wait for the login prompt as test end marker

The login prompt is printed after all the startup (test) scripts, wait
for that instead of "passed" marker. And only then check if test passed.
Before this patch there was a race: "passed" marker could be already
printed, but the final check would fail because login prompt wasn't
there yet.

Also, modify etc/issue in domU rootfs to avoid confusing the one from
domU with the dom0's one. Use the dom0 one as test end marker.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>