Support reporting live interface IP/netmask

Support reporting live interface IP/netmask

This patch adds the flag VIR_INTERFACE_XML_INACTIVE to
virInterfaceGetXMLDesc's flags. When it is*not* set (the default), the
live interface info will be returned in the XML (in particular, the IP
address(es) and netmask(s) will be retrieved by querying the interface
directly, rather than  reporting what's in the config file). The
backend of this is in netcf's ncf_if_xml_state() function.

* configure.in libvirt.spec.in: requires netcf >= 0.1.3
* include/libvirt/libvirt.h.in: adds flag VIR_INTERFACE_XML_INACTIVE
* src/conf/interface_conf.c src/interface/netcf_driver.c src/libvirt.c:
  update the parsing and backend routines accordingly
* tools/virsh.c: change interface edit to inactive definition and
  adds the inactive flag for interface dump