launch: libvirt: Allow port to be optional.

Actually if you don't specify the port, currently libvirt gives
an error:

  libguestfs: error: could not create appliance through libvirt: XML
  error: missing port for host [code=27 domain=20]

However I think that is a bug in libvirt:

  https://bugzilla.redhat.com/show_bug.cgi?id=965142
gluster: Don't force gluster port number to be specified.

Gluster can choose its own port number.
gluster: Force exportname to begin with a '/' character.
gluster: Note that client needs to run as root.
gluster: Normal port number (assuming 1 brick) is 24007.