[libvirt] Add support for qemu's guestfwd

This is an update of the patchset I posted yesterday. There have been a few changes in the 4th patch, which actually adds guestfwd support: Firstly, after discussion on IRC, it was agreed that 'vmchannel' is a poor name for the collection of implementations of private host<->guest channels, because it is the name of a specific implementation. Consequently the name has been changed to just 'channel'. Secondly, I've fixed a bunch of things I noticed while adding actual vmchannel support (patches to follow separately). Lastly, I've changed the relax-ng a bit to check the precise set of attributes for guestfwd. This change is also followed up in the vmchannel support. The patch now supports the following domain XML: <channel type='pipe'> <source path='/tmp/guestfwd'/> <target type='guestfwd' address='10.0.2.1' port='4600'/> </channel> It will output the following on the qemu command line for this: -chardev pipe,id=channel0,path=/tmp/guestfwd \ -net user,guestfwd=tcp:10.0.2.1:4600-chardev:channel0 Support for -chardev is as complete as the current character device support. Because of the legacy vmchannel, there was also some confusion about whether these patches are intended for merging. These patches aren't directly related to vmchannel, and are targeted at merging. Matt