tools/libxl: add direct_io_safe to check-xl-disk-parse

Add missing bool "direct_io_safe" to expected output. It was added by
Commit 6ec48cf4 ("libxl: introduce an option for disabling the
non-O_DIRECT workaround"), but check-xl-disk-parse was not updated.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
libxl: add option for discard support to xl disk configuration

Handle new option discard=on|off for disk configuration. It is supposed
to disable discard support if file based backing storage was
intentionally created non-sparse to avoid fragmentation of the file.

The option is a boolean and intended for the backend driver. A new
boolean property "discard-enable" is written to the backend node. An
upcoming patch for qemu will make use of this property. The kernel
blkback driver may be updated as well to disable discard for phy based
backing storage.

v3:
enable discard unconditionally by always writing discard-enable=1 to xenstore
fix typos in xl-disk-configuration.txt
update description in blkif.h, property should be ignored if unsupported
v2:
rename xenstore property from discard_enable to discard-enable
update description in xl-disk-configuration.txt
use libxl_defbool as type for discard_enable
update check-xl-disk-parse to use "<default>"
add LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE to libxl.h

Signed-off-by: Olaf Hering <olaf@aepfle.de>