CA-105789: Ensure the bridge has the correct MAC on first boot

Also, Use the interface name, not the bridge name when looking up MAC,
which is for firstboot and for when xe-reset-networking has been
executed.

Cherry pick of commits from xcp-networkd.git:
5b97592aaff07760693a416d100c253ebe456987
e23a6c1c71bf0a1c994ff546db3db6dc47640dc7

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>