bochs: ignore device if there isn't enougth memory

The qemu stdvga can be configured with a wide range of video memory,
from 1 MB to 256 MB (default is 16 MB).  In case it is configured
with only 1 or 2 MB it isn't really usable with bochsdrm, due to
depths other than 32bpp not being supported so that isn't enough
memory for a reasonable sized framebuffer.  So skip the device
and let vgacon or vesafb+fbcon handle the it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>