KVM: x86 emulator: centralize decoding of one-byte register access insns

Instructions like 'inc reg' that have the register operand encoded
in the opcode are currently specially decoded.  Extend
decode_register_operand() to handle that case, indicated by having
DstReg or SrcReg without ModRM.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/x86_emulate.h b/drivers/kvm/x86_emulate.h
index f03b128..e34868b 100644
--- a/drivers/kvm/x86_emulate.h
+++ b/drivers/kvm/x86_emulate.h
@@ -126,6 +126,7 @@
 	u8 rep_prefix;
 	u8 op_bytes;
 	u8 ad_bytes;
+	u8 rex_prefix;
 	struct operand src;
 	struct operand dst;
 	unsigned long *override_base;