Replace polling for active VMs with signalling by drivers

Replace polling for active VMs with signalling by drivers

Currently to deal with auto-shutdown libvirtd must periodically
poll all stateful drivers. Thus sucks because it requires
acquiring both the driver lock and locks on every single virtual
machine. Instead pass in a "inhibit" callback to virStateInitialize
which drivers can invoke whenever they want to inhibit shutdown
due to existance of active VMs.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>