lib: Implement attach-method unix:<path>

Allow connections to a Unix domain socket which is connected
(via virtio-serial) to a guestfsd running free in an existing
guest.

In order to use this you have to add the following element
to the libvirt XML:

  <channel type='unix'>
    <source mode='bind' path='/tmp/socket'/>
    <target type='virtio' name='org.libguestfs.channel.0'/>
  </channel>

(or perform the equivalent on the qemu command line).

Then in guestfish, you can do:

  guestfish \
    attach-method unix:/tmp/socket : \
    run : \
    ll /

(or any other commands as desired).