[libvirt] [PATCH 0/7] refactoring network xml parsing + net-update additions

Patches 1,2, and 4-7 refactor the network XML parsing functions to have a more uniform calling sequence for different subelements of <network>. This makes it simpler to write new virNetworkUpdate*() backend functions for those bits (because they're then more similar to existing functions.) Patch 3 actually adds some new backends for updating subelements that weren't previously implemented - <dns> host/srv/txt records. The intent is to followup on patched 4-7 with backends for updating <ip>, <forward>, and <bridge>; while I haven't gotten to those yet, I'd rather get these changes in sooner than later, both for more testing, and to make future backports simpler.