[PATCH 00 of 23] libxl: improved handling for default values in API

[Ian Campbell <ian.campbell@citrix.com>]

Introduce a mechanism for users of libxl to explicitly say "pick the
default for me".

To do this each field has a distinguished "init_val" and each struct
has a "_setdefault" method which sets (idempotently) the defaults for
fields which have not been set to an explicit value.

Previously I went through some contortions (with "timer_mode" in
particular but also with *_memkb fields) to try and arrange that this
distinguished value was the all zeroes bit pattern.

Instead of that pain this time I have arranged that the IDL supports
the specification the distinguished val for each type/field and we
autogenerate an _init function for each data type based on that.

Changes since last time:

* I have posted large parts of the previous series as
  * libxl: drop device_model_info
  * libxl: API updates + xl: JSON
  These have been committed thereby reducing the size of this series.
* _init function support as described above
* Dropped final patch to actually select stubdoms when possible --
  this needs more investigation/sanity checking.

This series has been lightly tested with PV domains (with and without
PVFB) and HVM domains (with and without stubdom).