virtio-vga: add '-vga virtio' support

Some convinience fluff:  Add support for '-vga virtio', also add
virtio-vga to the list of vga cards so '-device virtio-vga' will
turn off the default vga.

Written by Dave Airlie and Gerd Hoffmann.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/vl.c b/vl.c
index 9542095..2201e27 100644
--- a/vl.c
+++ b/vl.c
@@ -231,6 +231,7 @@
     { .driver = "isa-cirrus-vga",       .flag = &default_vga       },
     { .driver = "vmware-svga",          .flag = &default_vga       },
     { .driver = "qxl-vga",              .flag = &default_vga       },
+    { .driver = "virtio-vga",           .flag = &default_vga       },
 };
 
 static QemuOptsList qemu_rtc_opts = {
@@ -1884,6 +1885,11 @@
     return object_class_by_name("cgthree");
 }
 
+static bool virtio_vga_available(void)
+{
+    return object_class_by_name("virtio-vga");
+}
+
 static void select_vgahw (const char *p)
 {
     const char *opts;
@@ -1910,6 +1916,13 @@
             fprintf(stderr, "Error: VMWare SVGA not available\n");
             exit(0);
         }
+    } else if (strstart(p, "virtio", &opts)) {
+        if (virtio_vga_available()) {
+            vga_interface_type = VGA_VIRTIO;
+        } else {
+            fprintf(stderr, "Error: Virtio VGA not available\n");
+            exit(0);
+        }
     } else if (strstart(p, "xenfb", &opts)) {
         vga_interface_type = VGA_XENFB;
     } else if (strstart(p, "qxl", &opts)) {