vl: add missing display_remote++

We should also consider -display vnc= as setting up a remote display,
and not attempt to add another default one.

The display_remote++ in qemu_setup_display() isn't necessary at this
point, but is there for completeness and further usages of the variable.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1988
Fixes: commit 484629fc81 ("vl: simplify display_remote logic ")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

system/vl.c

@@ -1110,6 +1110,7 @@ static void parse_display(const char *p)
          */
         if (*opts == '=') {
             vnc_parse(opts + 1);
+            display_remote++;
         } else {
             error_report("VNC requires a display argument vnc=<display>");
             exit(1);
@@ -1359,6 +1360,7 @@ static void qemu_setup_display(void)
             dpy.type = DISPLAY_TYPE_NONE;
 #if defined(CONFIG_VNC)
             vnc_parse("localhost:0,to=99,id=default");
+            display_remote++;
 #endif
         }
     }