x86/msr: Dispatch Xen and Viridian MSRs from guest_{wr,rd}msr()

x86/msr: Dispatch Xen and Viridian MSRs from guest_{wr,rd}msr()

Despite the complicated diff in {svm,vmx}_msr_write_intercept(), it is just
the 0 case losing one level of indentation, as part of removing the call to
wrmsr_hypervisor_regs().

The case blocks in guest_{wr,rd}msr() use raw numbers, partly for consistency
with the CPUID side of things, but mainly because this is clearer code to
follow.  In particular, the Xen block may overlap with the Viridian block if
Viridian is not enabled for the domain, and trying to express this with named
literals caused more confusion that it solved.

Future changes with clean up the individual APIs, including allowing these
MSRs to be usable for vcpus other than current (no callers exist with v !=
current).

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>