vswitch: Don't choose Xen's FE:FF:FF:FF:FF:FF address as bridge MAC or DPID.

vswitch: Don't choose Xen's FE:FF:FF:FF:FF:FF address as bridge MAC or DPID.

Xen uses FE:FF:FF:FF:FF:FF as the mac address for every virtual interface.
If we have a bridge that has just virtual interfaces, then this will end
up as the bridge's MAC address and thus the datapath ID also.  So, instead,
use the bridge's randomly chosen MAC address if this case comes up.

This is probably not the end of this issue, since we probably want the
DPID of such bridges to be predictable.  Need to figure out how to do that.