tools/hotplug: set mtu from bridge also on vif interface

tools/hotplug: set mtu from bridge also on vif interface

Apply mtu size from bridge interface also in vif interface.
This depends on a kernel change which allows arbitrary mtu sizes until
the frontend driver has connected to the backend driver. Without this
kernel change, the vif mtu size will be limited to 1500 even with this
change to the vif-bridge script.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>