vdsm: introduce hostdev module

Hostdev module serves the purpose of encapsulating functions
related to reporting, acquiring and releasing host devices
(nodeDevices in libvirt terms)

Change-Id: I2bba96db5be180d00cb74fb89b10c5b09e5bd180
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32313
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>