[libvirt] Relax requirement of bridge source device

[Jim Fehlig]

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