amd: remove VIRT_SC_MSR_HVM synthetic feature

amd: remove VIRT_SC_MSR_HVM synthetic feature

Since the VIRT_SPEC_CTRL.SSBD selection is no longer context switched
on vm{entry,exit} there's no need to use a synthetic feature bit for
it anymore.

Remove the bit and instead use a global variable.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>