libxl: libxl__device_disk_local_attach return a new libxl_device_disk

libxl: libxl__device_disk_local_attach return a new libxl_device_disk

Introduce a new libxl_device_disk* parameter to
libxl__device_disk_local_attach, the parameter is allocated by the
caller. libxl__device_disk_local_attach is going to fill the new disk
with informations about the new locally attached disk.  The new
libxl_device_disk should be passed to libxl__device_disk_local_detach
afterwards.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>