x86/hvm: refactor set param

x86/hvm: refactor set param

To prevent leaking HVM params via L1TF and similar issues on a
hyperthread pair, let's load values of domains only after performing all
relevant checks, and blocking speculative execution.

For both get and set, the value of the index is already checked in the
outer calling function. The block_speculation calls in hvmop_get_param
and hvmop_set_param are removed, because is_hvm_domain already blocks
speculation.

Furthermore, speculative barriers are re-arranged to make sure we do not
allow guests running on co-located VCPUs to leak hvm parameter values of
other domains.

To improve symmetry between the get and set operations, function
hvmop_set_param is made static.

This is part of the speculative hardening effort.

Reported-by: Hongyan Xia <hongyxia@amazon.co.uk>
Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>