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

Adds tests and repairs some bugs in code.

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