vdsm: add functionality to detach host devices

Detach call (libvirt's dettach/detachFlags call) is required to
properly detach device from host. Dettaching device from the host means
unbinding it from it's driver and binding it to VFIO driver, which
serves as KVM device driver.

Change-Id: I530fed56831314286942b8afa62900208812bfb5
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32389
Reviewed-by: Dan Kenigsberg <danken@redhat.com>