x86emul: add read-modify-write hook

x86emul: add read-modify-write hook

In order to correctly emulate read-modify-write insns, especially
LOCKed ones, we should not issue reads and writes separately. Use a
new hook to combine both, and don't uniformly read the memory
destination anymore. Instead, DstMem opcodes without Mov now need to
have done so in their respective case blocks.

Also strip bogus _ prefixes from macro parameters when this only affects
lines which are being changed anyway.

In the test harness, besides some re-ordering to facilitate running a
few tests twice (one without and a second time with the .rmw hook in
place), tighten a few EFLAGS checks and add a test for NOT with memory
operand (in particular to verify EFLAGS don't get altered there).

For now make use of the hook optional for callers; eventually we may
want to consider making this mandatory.

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