Use full instance in virt driver volume usage

Right now, the virt driver method get_all_volume_usage() returns
a list of items, including the instance uuid. In order to pass
the results of this over to conductor (which will demand a full
instance), this patch converts the structure to include it.

It appears that only libvirt implements this method, and the change
is fairly trivial.

Related to blueprint no-db-compute-manager

Change-Id: Ia8001727b57442ef0ecc2c161201f1a617e199d4