libxl: introduce an option for disabling the non-O_DIRECT workaround

libxl: introduce an option for disabling the non-O_DIRECT workaround

Document and implement a new option that permits disk backends which
would otherwise have to avoid O_DIRECT (because of the network memory
lifetime bug) to use it anyway.  This is:
 direct-io-safe   in the xl domain disk config specification
 direct_io_safe   in the libxl disk API
 direct-io-safe   in the backend xenstore interface

Add a reference to xen/include/public/io/blkif.h in
docs/misc/vbd-interface.txt.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Felipe Franciosi <felipe@paradoxo.org>