vmd: properly advertise 640k base memory, not 636k. That was a holdover

vmd: properly advertise 640k base memory, not 636k. That was a holdover
from before we had seabios support (to fake a hole where the EBDA would
be), and seabios always assumes 640k low memory is available. Fixes a
problem where FreeDOS guests whose seabios placed the virtio ring too
close to 640k would crash vmd. tested on a variety of guest OS, with
and without seabios. no regressions seen.