Drop single use inheritance

There is only one class inheriting from Device, VmDevice. Device
has no instances, so it is better to drop it altogether in favor
or VmDevice, which is inherited from a sizeable amount of times.

Change-Id: If781ab20110874e71ba16b60d1d5511a54914979
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18351
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>