seabios: enable ide dma

QNX reportedly requires this to boot.
Should also speed up booting other guests.

Note: Upstream seabios defaults this to 'n' to due to known problems
on physical hardware (qemu not affected), and wouldn't flip the default
to 'y'.  So we adjust our local build config accordingly.

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