x86/hvm: Improvements to external users of decode_register()

x86/hvm: Improvements to external users of decode_register()

 * Rename to decode_gpr() to be more specific as to its purpose
 * Drop the highbyte encoding handling, as no users currently care, and it
   unlikely that future users would care.
 * Change to a static inline, returning an unsigned long pointer.

Doing so highlights that the "invalid gpr" paths in hvm_mov_{to,from}_cr()
were actually unreachable.  All callers already passed in-range GPRs, and
out-of-range GPRs would have hit the BUG() previously.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>