test_driver: make testDomainGetTime read time from vm-private data

test_driver: make testDomainGetTime read time from vm-private data

Until now, testDomainGetTime would always return the same fixed values
everytime it was called. By using domain-private data we can make this
API return the values previously set with testDomainSetTime, or use the
same old fixed values in case testDomainSetTime hasn't been called at all.

Signed-off-by: Ilias Stamatis <stamatis.iliass@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>