xen_disk: add discard support

Implement discard support for xen_disk. It makes use of the existing
discard code in qemu.

The discard support is enabled unconditionally. The tool stack may provide a
property "discard-enable" in the backend node to optionally disable discard
support.  This is helpful in case the backing file was intentionally created
non-sparse to avoid fragmentation.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>