CA-113612: Uncomment hvm_check_pvdriver in Domain.shutdown_wait_for_ack

The last two changes in this function (e8a7d8229c426a573bf461a0667993ed7644b28e
and 939cd11ecfa56daafc2daa1c3c81b1a2b97b0ffa) seem to have accidentally
commented-out an essential check.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #1462 from robhoes/ca113612

CA-113612: Uncomment hvm_check_pvdriver in Domain.shutdown_wait_for_ack