x86emul: address x86_insn_is_mem_{access,write}() omissions

x86emul: address x86_insn_is_mem_{access,write}() omissions

First of all explain in comments what the functions' purposes are. Then
make them actually match their comments.

Note that fc6fa977be54 ("x86emul: extend x86_insn_is_mem_write()
coverage") didn't actually fix the function's behavior for {,V}STMXCSR:
Both are covered by generic code higher up in the function, due to
x86_decode_twobyte() already doing suitable adjustments. And VSTMXCSR
wouldn't have been covered anyway without a further X86EMUL_OPC_VEX()
case label. Keep the inner case label in a comment for reference.

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