x86/spec-ctrl: Support BHI_DIS_S in order to mitigate BHI

x86/spec-ctrl: Support BHI_DIS_S in order to mitigate BHI

Introduce a "bhi-dis-s" boolean to match the other options we have for
MSR_SPEC_CTRL values.  Also introduce bhi_calculations().

Use BHI_DIS_S whenever possible.

Guests which are levelled to be migration compatible with older CPUs can't see
BHI_DIS_S, and Xen must fill in the difference to make the guest safe.  Use
the virt MSR_SPEC_CTRL infrastructure to force BHI_DIS_S behind the guest's
back.

This is part of XSA-456 / CVE-2024-2201.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
(cherry picked from commit 62a1106415c5e8a49b45147ca84d54a58d471343)