ide: enable single word DMA

ide: enable single word DMA

Windows XP doesn't enable DMA by default on dvd-rom devices emulated by QEMU.
Furthermore if I try to manually enable DMA using Device Manager, the
DMA mode chosen by Windows is inexplicably one of the few modes
unsupported by QEMU: single word mode 2.
Since on virtual hardware single word DMA is not really slower than
multi word DMA, it makes sense for QEMU to support single word dma as
well.

This patch does exactly this.  We also make sure that mdma is
deactivated when sdma is active and vice versa.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>