x86/emul: Improvements to internal users of decode_register()
Most users of decode_register() can be replaced with decode_gpr() right away.
For the few sites which do care about possibly using the legacy byteop
encoding, rename decode_register() to _decode_gpr() (to match its non-legacy
counterpart), and adjust its 'int highbyte_regs' parameter to the more correct
'bool legacy'.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: Jan Beulich <jbeulich@suse.com>