[libvirt] Relax requirement of bridge source device

Hi All, I wanted to get folks thoughts on relaxing the requirement for bridge source device in network interface of domain XML, e.g. allowing <interface type="bridge"> <mac address="aa:bb:cc:cc:ee:ff"/> </interface> Currently, virDomainNetDefParseXML() in src/domain_conf.c will fail such configuration. Since this is common code between backends I'm not sure how allowing this config will affect the various virtualizers. Xen's vif-bridge script will try to find a bridge if one is not specified, which perhaps is dubious behavior, but nonetheless convenient in migration scenarios where the target host may have a different bridge name (br1 vs br0 for example). IIRC, qemu's qemu-ifup script is not as forgiving in the absence of a bridge name. I would be happy to provide a patch if relaxing the source device requirement is acceptable. Thanks, Jim