libxl: properly set default of discard_enable

Initialze discard_enable properly. This avoids a crash if a
libxl_device_disk with an uninitialized discard_enable is passed to
device_disk_add. Up to now only xl initialized discard_enable in its
config parser. External users of libxl, such as libvirt, do not need to
provide a default value.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Merge branch 'staging' of ssh://xenbits.xen.org/home/xen/git/xen into staging