ide.c make write cacheing controllable by guest

ide.c make write cacheing controllable by guest

The attached patch implements the ATA write cache feature.  This
enables a guest to control, in the standard way, whether disk writes
are immediately committed to disk before the IDE command completes, or
may be buffered in the host.

In this patch, by default buffering is off, which provides better
reliability but may have a performance impact.  It would be
straightforward to change the default, or perhaps offer a command-line
option, if that would be preferred.

This patch is derived from one which was originally submitted to the
Xen tree by Rik van Riel <riel@redhat.com>.