core: use previously used address of balloon device

We should pass the previously used address of balloon device when
running VM so it won't be detected as a new device and trigger
the 'install driver for new device' dialog in the guest

Change-Id: Ie4b93a14f395c3e7bdb83dc0a25dbda0546e4e92
Bug-Url: https://bugzilla.redhat.com/1028387
Signed-off-by: Arik Hadas <ahadas@redhat.com>