Put network.rng contents inside <grammar> for uniformity with other RNGs

Put network.rng contents inside <grammar> for uniformity with other RNGs

All the other RNG files in libvirt are enclosed within <grammar>. This
commit makes the syntactical changes necessary to make network.rng fit
that pattern. (This is the first step in adding some data type
definitions to network.rng for more exact validation of IP and MAC
addresses).

Formatting changes (indentation) will be done in a subsequent commit,
so that actual changes to the code won't be obscured by whitespace.