x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL

x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL

Hardware maintains both host and guest versions of MSR_SPEC_CTRL, but guests
run with the logical OR of both values.  Therefore, in principle we want to
clear Xen's value before entering the guest.  However, for migration
compatibility (future work), and for performance reasons with SEV-SNP guests,
we want the ability to use a nonzero value behind the guest's back.  Use
vcpu_msrs to hold this value, with the guest value in the VMCB.

On the VMEntry path, adjusting MSR_SPEC_CTRL must be done after CLGI so as to
be atomic with respect to NMIs/etc.

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