arm-autonomy, xenguest-network-bridge: Set bridge members per machine

By default, XENGUEST_NETWORK_BRIDGE_MEMBERS should be empty and only be
set per machine.

Change-Id: I67c56847eb785279c28757a6052f092c838babe0
Issue-Id: SCM-995
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>