libxl: handle the return value of wait_for_dev_destroy select

libxl: handle the return value of wait_for_dev_destroy select

Handle the return value of the select call inside wait_for_dev_destroy
properly, and return 0 if a device is removed, or ERROR_* if a timeout
or error happened. Use the return value of wait_for_dev_destroy inside
libxl__device_remove to properly return from that function.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>