Do not probe for power mgmt capabilities in lxc emulator

Do not probe for power mgmt capabilities in lxc emulator

It fails after 30 seconds with this error:
error : virDBusCall:1429 : error from service: CanSuspend:
Did not receive a reply. Possible causes include: the remote
application did not send a reply, the message bus security
policy blocked the reply, the reply timeout expired, or the
network connection was broken.

Only probe for the power mgmt capabilities when driver is non-NULL.
This speeds up domain startup by 30 seconds.

https://bugzilla.redhat.com/show_bug.cgi?id=1159227