x86/vmce: Dispatch vmce_{rd,wr}msr() from guest_{rd,wr}msr()

x86/vmce: Dispatch vmce_{rd,wr}msr() from guest_{rd,wr}msr()

... rather than from the default clauses of the PV and HVM MSR handlers.

This means that we no longer take the vmce lock for any unknown MSR, and
accesses to architectural MCE banks outside of the subset implemented for the
guest no longer fall further through the unknown MSR path.

The bank limit of 32 isn't stated anywhere I can locate, but is a consequence
of the MSR layout described in SDM Volume 4.

With the vmce calls removed, the hvm alternative_call()'s expression can be
simplified substantially.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>