vdsm: add functionality to reattach host devices

Release call (libvirt's reAttach call) is mandatory to properly
reattach device back to host, unbinding it from VFIO driver and
binding it back to host driver.

Change-Id: I77000c76ece87bf7335045429eab207a89badae9
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32391
Reviewed-by: Dan Kenigsberg <danken@redhat.com>