libvirt: fix blockinfo get_device_name helper

Currently all the methods in libvirt's blockinfo module will either
be called with block device objects (when attempting to default device
names)  or with a DriverBlockDevice subclass (all other operations in
the virt driver that expect such format to be passed).

This patch updates the get_device_name method to honor the above
situation introduced by I055651b5fd415092787d9487a69467a9bbcf073e.

Closes-bug: #1290302
Change-Id: I7099ea4502da2108c4bcba530a8ace173d2dafc9
Merge "libvirt: fix blockinfo get_device_name helper"