KVM: x86: rename EMULATE_DO_MMIO

The next patch will reuse it for other userspace exits than MMIO,
namely debug events.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 3a9493a..2c1bb95 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4182,7 +4182,7 @@
 	switch (er) {
 	case EMULATE_DONE:
 		return 1;
-	case EMULATE_DO_MMIO:
+	case EMULATE_USER_EXIT:
 		++vcpu->stat.mmio_exits;
 		/* fall through */
 	case EMULATE_FAIL: