engine: Store boot order after plug/unplug vNIC

Store the updated boot order in DB after the plugging/unplugging is
performed.

Change-Id: I1fa77fbf727f4df96d5bfa3a2aaa4fc1c9188295
Bug-Url: https://bugzilla.redhat.com/907155
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>