x86/HVM: cache emulated instruction for retry processing

x86/HVM: cache emulated instruction for retry processing

Rather than re-reading the instruction bytes upon retry processing,
stash away and re-use what we already read. That way we can be certain
that the retry won't do something different from what requested the
retry, getting once again closer to real hardware behavior (where what
we use retries for is simply a bus operation, not involving redundant
decoding of instructions).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>