interface: allow multiple IPv4 addresses + dhcp on a single interface
    
    As of netcf-0.2.8, netcf supports configuring multipl IPv4 addresses,
    as well as simultaneously configuring dhcp and static IPv4 addresses,
    on a single interface. This patch updates libvirt's interface.rng to
    allow such configurations.
    
    This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1223688