engine: VmPayload needs to inherit from VmDevice

It is represent a device and we need the Address and the Id for stable
devices.

Change-Id: Ieabf99eaaff0d6e68a0521af3df43a774ca31590
Bug-Url: https://bugzilla.redhat.com/1138314
Signed-off-by: Shahar Havivi <shaharh@redhat.com>