libxl: fix libxl__device_disk_from_xs_be to parse backend domid

libxl: fix libxl__device_disk_from_xs_be to parse backend domid

libxl__device_disk_from_xs_be was ignoring the backend domid, setting
it to 0 by default. Fix this by parsing the backend disk path in order
to fetch the backend domid.

This fixes the issue reported when trying to block-detach disks that
have it's backend on a driver domain.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: G.R. <firemeteor@users.sourceforge.net>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>