xenapi: 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: Ie6b587d3f89202a6987291bfaea88290241651f2
Merge "xenapi: virt unrescue method now supports objects"