KVM: use a more sensible error number when debugfs directory creation fails

I don't know if this was due to cut and paste, or somebody was really
using a D20 to pick the error code for kvm_init_debugfs as suggested by
Linus (EFAULT is 14, so the possibility cannot be entirely ruled out).

In any case, this patch fixes it.
Reported-by: NTim Gardner <tim.gardner@canonical.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3091,7 +3091,7 @@ static const struct file_operations *stat_fops[] = {
 
 static int kvm_init_debug(void)
 {
-	int r = -EFAULT;
+	int r = -EEXIST;
 	struct kvm_stats_debugfs_item *p;
 
 	kvm_debugfs_dir = debugfs_create_dir("kvm", NULL);