vdsm: initial split of vm devices

Initial boostrapping of devices. This is effort to move
things out of vm.py to their modules in order to improve readability
and isolate functionality.

Change-Id: I63b602851c17697259e86dfad3a9063447c57e50
Signed-off-by: Martin Polednik <mpolednik@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29250
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>