libvirt: virt unrescue method now supports objects

Commit 65ad230af5ece020822fa963dd30fd641a54ae91 ensured
that the compute API and manager are now getting objects
for the unrescue method.

This patch adds support to the virt driver code.

Change-Id: I5cca3fa997042b164c21fe2a13c2311f4b18ec06
Merge "libvirt: virt unrescue method now supports objects"