network: backend for virNetworkUpdate of dhcp range

network: backend for virNetworkUpdate of dhcp range

The dhcp range element is contained in the <dhcp> element of one of a
network's <ip> elements. There can be multiple <range>
elements. Because there are only two attributes (start and end), and
those are exactly what you would use to identify a particular range,
it doesn't really make sense to modify an existing element, so
VIR_NETWORK_UPDATE_COMMAND_MODIFY isn't supported for this section,
only ADD_FIRST, ADD_LAST, and DELETE.

Since virsh already has support for understanding all the defined
sections, this new backend is automatically supported by virsh. You
would use it like this:

  virsh net-update mynet add ip-dhcp-range \
        "<range start='1.2.3.4' end='1.2.3.20'/>" --live --config

The bridge driver also already supports all sections, so it's doing
the correct thing in this case as well - since the dhcp range is
placed on the dnsmasq commandline, the bridge driver recreates the
dnsmasq commandline, and re-runs dnsmasq whenever a range is
added/deleted (and AFFECT_LIVE is specified in the flags).