[libvirt] Add support for legacy -vmchannel option

[Matthew Booth]

These patches add support for the legacy qemu -vmchannel option. The following
domain xml:

<channel type='unix'>
  <source mode='bind' path='/tmp/vmchannel'/>
  <target type='vmchannel' deviceid='0200'/>
</channel>

becomes:

-vmchannel di:0200,unix:/tmp/vmchannel,server,nowait

I don't expect these to be merged, but I would appreciate a review.

Thanks,

Matt