[libvirt] Add support for qemu's guestfwd

[Matthew Booth]

This series adds support for a vmchannel using qemu's guestfwd. I'll be
following up with support for virtio-serial and RHEL5's vmchannel.

A vmchannel is specified as:

<vmchannel type='pipe'>
  <source path='/tmp/vmchannel'/>
  <target type='guestfwd' address='10.0.2.1' port='4600'/>
</vmchannel>

There are a couple of minor complications. Firstly, the existing chrdev
structure assumes that all front-ends take only a port. There's a bit of code
churn adding a second union to this struct to allow for different data for
different backends.

Secondly, it turns out that the existing syntax for character devices doesn't
really work for guestfwd. Specifically it won't allow common options to be
added. This is fixed in qemu git with the new -chardev syntax. Consequently,
this patch only adds support for guestfwd using -chardev. We add detection for
-chardev, and a new internal utility function to output -chardev command lines
for all existing character device backends.

Matt