libxl: allow only 'ethernet' and 'bridge' interfaces, allow script there
libxl: allow only 'ethernet' and 'bridge' interfaces, allow script there

Actually only those interface types are handled correctly so reject
others instead of ignoring settings (i.e. treating as bridge/ethernet
anyway).
Also allow <script/> in 'ethernet' (which should be the only
script-allowing type). Keep <script/> allowed in bridge to be compatible
with legacy 'xen' driver.

Signed-off-by: Marek Marczykowski-Górecki <b37867d4e08020cf5bb17f05a2c036fbc82d5481@invisiblethingslab.com>

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