virDomainObjCheckActive() returns -1 if domain is not active, not 0.

Fixes cb50436c "libxl: implement virDomainPM* functions"
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>