vz: fixed race in vzDomainAttach/DettachDevice
While dettaching/attaching device in OpenStack, nova
calls vzDomainDettachDevice twice, because the update of the internal
configuration of the ct comes a bit latter than the update event.
As the result, we suffer from the second call to dettach the same device.
Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com>