libxl: introduce helper to initialise Dom0

libxl: introduce helper to initialise Dom0

This small helper is responsible for generating Dom0 JSON config
stub and writing Dom0 xenstore entries. This helpers subsumes two calls
to xenstore-write in xencommons script.

Dom0 UUID is intentionally left untouched, so it is always all
zeros.  This makes sure that we don't leak Dom0 stubs across rebooting.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- folded in incremental patch for *BSD ]