stubdom: make non-VNC + stdvga work

diff -r ba6c8c43a125 tools/ioemu/hw/vga.c
@@ -2144,6 +2144,11 @@
     /* and page-aligned for PVFB memory sharing */
     s->vram_ptr = s->vram_alloc = qemu_memalign(TARGET_PAGE_SIZE, vga_ram_size);
 
+#ifdef CONFIG_STUBDOM
+    if (!cirrus_vga_enabled)
+        xenfb_pv_display_start(s->vram_ptr);
+#endif
+
     s->vram_offset = vga_ram_offset;
     s->vram_size = vga_ram_size;
     s->ds = ds;