x86emul: correct behavior for single iteration REP INS/OUTS

x86emul: correct behavior for single iteration REP INS/OUTS

The initial operation done on these paths may raise an exception (for
->read_io() that's possible only on the PV path, when the I/O port
access check has been deferred). We have to suppress put_rep_prefix()
updating rCX in that case. From an abstract perspective this also
applies to RETRY being returned.

Reported-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>