Several bugfixes have reduced the utility of this variable from it's original
purpose, and now all it does is aid in the setup of SCF_ist_wrmsr.
Simplify the logic by drop the variable, and doubling up the setting of
SCF_ist_wrmsr for the PV and HVM blocks, which will make the AMD SPEC_CTRL
support easier to follow. Leave a comment explaining why SCF_ist_wrmsr is
still necessary for the VMExit case.
No functional change.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: Jan Beulich <jbeulich@suse.com>