virt: pass instance object to block_stats & get_instance_disk_info

The block_stats and get_instance_disk_info methods are the last
two virt driver methods which do not take a full Nova Instance
object. They are trivially convertable.

Blueprint: libvirt-driver-class-refactor
Change-Id: I87ef651383c9b4ab3abe4bf8a5c49501e353abc4
Merge "virt: pass instance object to block_stats & get_instance_disk_info"