vl.c: convert *vga_enabled functions to QOM

And get rid of qdev_exists().

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/vl.c b/vl.c
index 8d305ca..bf9cdf8 100644
--- a/vl.c
+++ b/vl.c
@@ -1700,17 +1700,18 @@
 
 static bool vga_available(void)
 {
-    return qdev_exists("VGA") || qdev_exists("isa-vga");
+    return object_class_by_name("VGA") || object_class_by_name("isa-vga");
 }
 
 static bool cirrus_vga_available(void)
 {
-    return qdev_exists("cirrus-vga") || qdev_exists("isa-cirrus-vga");
+    return object_class_by_name("cirrus-vga")
+           || object_class_by_name("isa-cirrus-vga");
 }
 
 static bool vmware_vga_available(void)
 {
-    return qdev_exists("vmware-svga");
+    return object_class_by_name("vmware-svga");
 }
 
 static void select_vgahw (const char *p)