[libvirt] virtio-serial support

[Matthew Booth]

These patches rebase my previously posted virtio-serial patches, add support for
max_ports and vectors, and allow multiple virtio-serial devices to be used. I
wasn't going to bother with the latter, however it turns out this can be useful
if the user wants more than 31 virtserialports.

N.B. These apply on top of 2 previously posted patches. Specifically "Fix whitespace in domain.rng" and "Remove unused functions from domain_conf".