libxl: validate scheduler parameters

libxl: validate scheduler parameters

This was previously done by xl itself however the domain was not
created at that point so there was no domid to check. This happened to
work on first boot because xl's global domid was initialised to zero
so we would (incorrectly) validate the new domain to be against
domain0. On reboot though we would try to use the old domain's id and
fail.

sched_params_valid is moved and gains a gc+domid parameters and
s/ctx/CTX/. The call is placed after
libxl__domain_build_info_setdefault in the create path, because
set_defaults doesn't have access to the domid and there are other
callers which don't even have a domid to give it.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>