Revert "x86/msr: drop compatibility #GP handling in guest_{rd,wr}msr()"

Revert "x86/msr: drop compatibility #GP handling in guest_{rd,wr}msr()"

In hindsight, this was a poor move.  Some of these MSRs require probing for,
cause unhelpful spew into xl dmesg, or cause spew from unit tests explicitly
checking behaviour.

This restores behaviour close to that of Xen 4.14, meaning in particular
that for all of the MSRs getting re-added explicitly a #GP fault will get
raised irrespective of the new "msr_relaxed" setting.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
Acked-by: Ian Jackson <iwj@xenproject.org>