engine: Use VmDevice.id to identify the vm device instance

The VmDevice.Id is sufficient to identify the existence of
the device within a map, rather using the entire device
which could be modified by other flows in the system (i.e.
update boot sequence).

Change-Id: Id0a976e31e97c1f250e29db9166e60ccba4b4a94
Bug-Url: https://bugzilla.redhat.com/1105842
Bug-Url: https://bugzilla.redhat.com/1136031
Signed-off-by: Moti Asayag <masayag@redhat.com>
(cherry picked from commit 06e1857bf4b2a3f8c81c4399c5799e9beb6e67c7)