vm: narrow down try/except block in releaseVm

In releaseVm, we have a too wide try/except block;
only self._dom.destroy() can actually raise libvirtError,
so this patch narrows down the block size.

Change-Id: I9c2b60d8f490f9559edad429baf3989e98d05cf6
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28024
Reviewed-by: Dan Kenigsberg <danken@redhat.com>