libxl: Rerun bison

This updates libxlu_cfg_y.[ch] to code generated by bison from
Debian wheezy (1:2.5.dfsg-2.1 i386).

There should be no functional change since there is no change to the
source file, but we will inherit bugfixes and behavioural changes from
the new version of bison.  So this is more a matter of hope than
knowledge.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
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>