x86/spec_ctrl: Split X86_FEATURE_SC_MSR into PV and HVM variants

x86/spec_ctrl: Split X86_FEATURE_SC_MSR into PV and HVM variants

In order to separately control whether MSR_SPEC_CTRL is virtualised for PV and
HVM guests, split the feature used to control runtime alternatives into two.
Xen will use MSR_SPEC_CTRL itself if either of these features are active.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
(cherry picked from commit fa9eb09d446a1279f5e861e6b84fa8675dabf148)

Gbp-Pq: Name x86spec_ctrl-split-x86_feature_sc_msr-in.patch