CI: wait for Xen to start before waiting for test to complete

CI: wait for Xen to start before waiting for test to complete

Add additional stage in console output parsing - wait for first message
from Xen. The message is defined via BOOT_MSG variable. This has two
effects:
- distinguishes failing Xen to load at all from later test failures
- resets timeout when Xen starts loading

The latter is especially relevant for hardware tests where firmware +
network boot may take some time before Xen starts booting. The two-stage
timeout is more robust solution than increasing the overall timeout.
The issue has been observed on some dom0pvh-hvm jobs, at least on
runners hw3 and hw11. This patch is a first stage before qubes-x86-64.sh
is switched to use expect in the next stage.

While at it, consistently use 'expect -re' for all matches. This
especially allows matching newlines ("\n"), which will become relevant
in the next patch. And document variables used in console.exp.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>