XenAPI raise InstanceNotFound in _get_vm_opaque_ref

If _get_vm_opaque_ref doesn't find an instance it should raise a
specific InstanceNotFound exception rather than the general NotFound
exception.  Callers of this method may try to catch InstanceNotFound or
NotFound and now both will work.

This fix will prevent a traceback during the _sync_power_states periodic
task which is expecting an InstanceNotFound exception.

Change-Id: Ia28fe33d69520806dab878c843fe435e018a20de
Closes-Bug: 1226756
Merge "XenAPI raise InstanceNotFound in _get_vm_opaque_ref"