Add resource tracking to unshelve_instance()

When unshelving an offloaded instance, a new host is scheduled for
the instance. However, the call to unshelve the instance on the new
host does not go through the resource tracker, which may lead to
over-allocation of the hosts resources.

This change adds resource tracking to unshelve_instance().

Change-Id: I682713c8ebc6276ac49822db5c8efc373d5b8f4d
Closes-Bug: #1241017