Virt driver logging improvements

Right now, there's no clue in nova logs whether or not we're still
polling nodes in ironic for state changes when provisioning or
unprovisioning. Let's add some log messages for this.

Also, make get_available_resources() log the number of nodes along with
the node uuids.

Change-Id: Ic83f4389ba1ca576deb87f46cde84ce414c5baeb
Merge "Virt driver logging improvements"