Add guestfs_add_domain 'live' flag.

This optional flag controls whether this API call will try to connect
to a running virtual machine 'guestfsd' process.

If the flag is given and the virtual machine is running, then the
libvirt XML is parsed looking for a suitable <channel> element, and
'guestfs_set_attach_method' is called with the corresponding
virtio-serial socket path.