libxl: set stub domain size based on VRAM size

Allocate additional memory to the stub domain for qemu-traditional if
more than 4 MB is assigned to the video adapter to avoid out of memory
condition for QEMU.

For LIBXL_VGA_INTERFACE_TYPE_STD + CIRRUS video_memkb is always at
least 4MB. However if LIBXL_VGA_INTERFACE_TYPE_STD is selected the
video_memkb defaults to 0, resulting in a stubdom which is 4MB smaller
than before. It seems unlikely that VGA disable would require more
memory then the other options, so this should be ok.

Signed-off-by: Eric Shelton <eshelton@pobox.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- added second paragraph of commit message ]

tools/libxl/libxl_dm.c

@@ -1096,7 +1096,8 @@ void libxl__spawn_stub_dm(libxl__egc *egc, libxl__stub_dm_spawn_state *sdss)
     libxl_domain_build_info_init_type(&dm_config->b_info, LIBXL_DOMAIN_TYPE_PV);
 
     dm_config->b_info.max_vcpus = 1;
-    dm_config->b_info.max_memkb = 32 * 1024;
+    dm_config->b_info.max_memkb = 28 * 1024 +
+        guest_config->b_info.video_memkb;
     dm_config->b_info.target_memkb = dm_config->b_info.max_memkb;
 
     dm_config->b_info.u.pv.features = "";