x86emul: also test decoding and mem access / write logic

x86emul: also test decoding and mem access / write logic

x86emul_is_mem_{access,write}() (and their interaction with
x86_decode()) have become sufficiently complex that we should have a way
to test this logic. Start by covering legacy encoded GPR insns, with the
exception of a few the main emulator doesn't support yet (left as
comments in the respective tables, or about to be added by subsequent
patches). This has already helped spot a few flaws in said logic,
addressed by (revised) earlier patches.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>