x86emul: CMPXCHG{8,16}B ignore prefixes

x86emul: CMPXCHG{8,16}B ignore prefixes

This removes 0F C7 from the list of two-byte opcodes treating prefixes
66, F3, and F2 as opcode extensions. We better manually handle this in
the opcode specific code:
- CMPXCHG8B ignores all these prefixes (its handling is being adjusted
  accordingly, with a respective test case added as well, to avoid
  re-introducing the subject of XSA-200),
- RDRAND/RDSEED (support to be added subsequently) honor 66, but treat
  F3 and F2 as opcode extensions (resolving to RDPID in the RDSEED
  case, which in turn ignores 66).

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