The new listenNetwork attribute needs to learn an IP address based on a
named network. This patch provides a function networkGetNetworkAddress
which provides that.

Some networks have an IP address explicitly in their configuration
(ie, those with a forward type of "none", "route", or "nat"). For
those, we can just return the IP address from the config.

The rest will have a physical device associated with them (either via
<bridge name='...'/>, <forward ... dev='...'/>, or possibly via a pool
of interfaces inside the network's <forward> element) and we will need
to ask the kernel for a current IP address of that device (via the
newly added ifaceGetIPAddress)

If networkGetNetworkAddress encounters an error while trying to learn
the address for a network, it will return -1. In the case that libvirt
has been compiled without the network driver, the call is a macro
which reduces to -2. This allows differentiating between a failure of
the network driver, and its complete absence.