libxl: introduce a way to mark fields as deprecated in the idl

libxl: introduce a way to mark fields as deprecated in the idl

The deprecation involves generating a function that copies the
deprecated fields into it's new location if the new location has not
been set.

The fields that are going to be shared between PVH and HVM or between
PVH and PV are moved to the top level of libxl_domain_build_info, and
the old locations are marked as deprecated.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>