KVM: x86 emulator: fix repne/repnz decoding

The repnz/repne instructions must set rep_prefix to 1 like rep/repe/repz.

This patch correct the disk probe problem met with OpenBSD.

This issue appears with commit e70669ab


because before it, the decoding was done internally to kvm and after it
is done by x86_emulate.c (which doesn't do it correctly).

Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>