spice: add unix-path in .vv file support
    
    This will allow to connect to a Spice server using a unix socket path,
    for example:
    
    [virt-viewer]
    type=spice
    unix-path=/var/run/user/1000/qemu/test/spice.sock
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Reviewed-by: Christophe Fergeau <cfergeau@redhat.com>