x86/msr: Split MSR_SPEC_CTRL handling

x86/msr: Split MSR_SPEC_CTRL handling

In order to fix a VT-x bug, and support MSR_SPEC_CTRL on AMD, move
MSR_SPEC_CTRL handling into the new {pv,hvm}_{get,set}_reg() infrastructure.

Duplicate the msrs->spec_ctrl.raw accesses in the PV and VT-x paths for now.
The SVM path is currently unreachable because of the CPUID policy.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 6536688439dbca1d08fd6db5be29c39e3917fb2f
master date: 2022-01-20 16:32:11 +0000