x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

In some cases, writes to MSR_SPEC_CTRL do not have interesting side effects,
and we should implement lazy context switching like we do with other MSRs.

In the short term, this will be used by the SVM infrastructure, but I expect
to extend it to other contexts in due course.

Introduce cpu_info.last_spec_ctrl for the purpose, and cache writes made from
the boot/resume paths.  The value can't live in regular per-cpu data when it
is eventually used for PV guests when XPTI might be active.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>