interface: allow multiple IPv4 addresses + dhcp on a single interface

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