This patch fills in the first implementation for one of the
virNetworkUpdate sections. With this code, you can now add/delete/edit
<host> entries in a network's <ip> address <dhcp> element (by
specifying a section of VIR_NETWORK_SECTION_IP_DHCP_HOST).

If you pass in a parentIndex of -1, the code will automatically find
the one ip element that has a <dhcp> section and make the updates
there. Otherwise, you can specify an index >= 0, and libvirt will look
for that particular instance of <ip> in the network, and modify its
<dhcp> element. (This currently isn't very useful, because libvirt
only supports having dhcp information on a single IP address, but that
could change in the future).

When adding a new host entry
(VIR_NETWORK_UPDATE_COMMAND_ADD_(FIRST|LAST)), the existing entries
will be compared to the new entry, and if any non-empty attribute
matches, the add will fail. When updating an existing entry
(VIR_NETWORK_UPDATE_COMMAND_MODIFY), the mac address or name will be
used to find the existing entry, and other fields will only be updated
(note there is some potential for ambiguity here if you specify the
mac address from one entry and the name from another).  When deleting
an existing entry (VIR_NETWORK_UPDATE_COMMAND_DELETE), all non-empty
attributes in the supplied xml arg will be compared - all of them must
match before libvirt will delete the host.

The xml should be a fully formed <host> element as it would appear in
a network definition, e.g. "<host mac=00:11:22:33:44:55 ip=10.1.23.22
name='testbox'/>" (when adding/updating, ip and one of mac|name is
required; when deleting, you can specify any one, two, or all
attributes, but they all must match the target element).

As with the update of any other section, you can choose to affect the
live config (with flag VIR_NETWORK_UPDATE_AFFECT_LIVE), the persistent
config (VIR_NETWORK_UPDATE_AFFECT_CONFIG), or both. If you've chosen
to affect the live config, those changes will take effect immediately,
with no need to destroy/restart the network.

An example of adding a host entry:

   virNetworkUpdate(net, VIR_NETWORK_UPDATE_COMMAND_ADD_LAST,
                     VIR_NETWORK_SECTION_IP_DHCP_HOST, -1,
                    "<host mac='00:11:22:33:44:55' ip='192.168.122.5'/>",
                    VIR_NETWORK_UPDATE_AFFECT_LIVE
                    | VIR_NETWORK_UPDATE_AFFECT_CONFIG);

To delete that same entry:

   virNetworkUpdate(net, VIR_NETWORK_UPDATE_COMMAND_DELETE,
                    VIR_NETWORK_SECTION_IP_DHCP_HOST, -1,
                    "<host mac='00:11:22:33:44:55'/>",
                    VIR_NETWORK_UPDATE_AFFECT_LIVE
                    | VIR_NETWORK_UPDATE_AFFECT_CONFIG);

(you could also delete it by replacing "mac='00:11:22:33:44:55'" with
"ip='192.168.122.5'".)