virt: Adds multihost test migration with GPU in VGA mode.

Guest has to be prepared for running with qlx
driver. Drivers have to be properly installed
before starting of the test. Otherwise the test
could FAIL.
tests steps:
    1) starts windows XP guest on first host
    2) try login to guest
    3) starts presubtest steps with
       stepsfile = WinXP-set-VGA-mode.steps
       enable VGA mode on GPU
    4) after presubtest finished starts migration
       to host2
    5) after migration starts postsubtests with
       stepfile = WinXP-reset-VGA-mode.steps

Signed-off-by: Jiří Župka <jzupka@redhat.com>