libxl: set stub domain size based on VRAM size

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 ]