x86/MSR: introduce MSR access split/fold helpers

x86/MSR: introduce MSR access split/fold helpers

This is in preparation of eliminating the mis-naming of 64-bit fields
with 32-bit register names (eflags instead of rflags etc). Use the
guaranteed 32-bit underscore prefixed names for now where appropriate.

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