x86/spec-ctrl: Make svm_vmexit_spec_ctrl conditional

x86/spec-ctrl: Make svm_vmexit_spec_ctrl conditional

The logic was written this way out of an abundance of caution, but the reality
is that AMD parts don't currently have the RAS-flushing side effect, nor do
they intend to gain it.

This removes one WRMSR from the VMExit path by default on Zen2 systems.

Fixes: 614cec7d79d7 ("x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>