[libvirt] [PATCH 0/3] Add proper validation of IP/MAC addresses to network.rng

I needed to add more elements to network.rng, and modify the possible values for some existing elements, but noticed that there was currently no validation of IP addresses or MAC addresses. As a first stpe in my modifications, I'm bringing network.rng's validation/format more in line with what's in, eg, interface.rng. I'll be adding the new elements later, in a complete patcheset to add IPv6 support, but since these patches stand on their own, I figured I'd get them out of the way first. The first patch puts the entire definition of network inside a <grammer>, which seemed like the right thing to do in order to be able to define refs for ipv4-addr/mac-addr outside the element def itself. I copied the way that it's done in other libvirt RNG files (not sure why this one wasn't like that already). Although this changed the nesting of almost the entire file, I left the indentation of existing lines as-is so the actual change wouldn't be obscured. The 2nd patch looks very long and confused, but it actually is just whitespace changes (as verified by diff -b). The 3rd patch adds the refs for ipv4-addr & mac-addr, and makes use of them. networkschematest continues to succeed after these changes, so it looks like it's all correct.