make write cacheing controllable by guest

make write cacheing controllable by guest

This 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> and includes code to save
the write_cache setting from Samuel Thibault.

From: Rik van Riel <riel@redhat.com>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>
Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>