libxl: limit support for specifying an interface script

libxl: limit support for specifying an interface script

Generally, <interface> ... <script> is only supported for
type='ethernet'.  Due to the long and pervasive use of

  <interface type='bridge'>
    ...
    <script path='foo'/>
  </interface>

in Xen domain configuration, it was agreed to allow the use
of <script> with type='bridge' for backwards compatibility.  See
the following discussion thread

http://www.redhat.com/archives/libvir-list/2013-April/msg00755.html

This patch limits the use of <script> to interface types ethernet
and bridge, raising an unsupported config error if <script> is
specified for all other interface types.

While at it, use VIR_ERR_CONFIG_UNSUPPORTED instead of
VIR_ERR_INTERNAL_ERROR when reporting unsupported interface types.