libxl: add option for discard support to xl disk configuration

libxl: add option for discard support to xl disk configuration

Handle new boolean option discard/no-discard 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 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.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>