Introduce -display none

New option -display none. This option differs from -nographic by not
trying to take control of stdio etc. but instead behaves as if a
graphics display is enabled, except that it doesn't show one.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/vl.c b/vl.c
index dd026aa..2da2136 100644
--- a/vl.c
+++ b/vl.c
@@ -1618,6 +1618,8 @@
         fprintf(stderr, "Curses support is disabled\n");
         exit(1);
 #endif
+    } else if (strstart(p, "none", &opts)) {
+        display = DT_NONE;
     } else {
     invalid_display:
         fprintf(stderr, "Unknown display type: %s\n", p);