vmDevices: add __slots__ to devices

VDSM devices are created using setattr over **kwargs, making the
initialization implicit. Implementing __slots__ does define which
attributes does the device accept and also slightly improves memory footprint.

Change-Id: I6e8dadabdd02d3b44606f215c4bc7b7e306a591a
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21036
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>