Invalid max_roi in (prefetched) loader mode

Related to bliss/bliss!4280 (comment 159430)

import tango
ctl = tango.DeviceProxy("id00/limaccds/samsim")
det = tango.DeviceProxy("id00/simulator/samsim")
det.mode = "LOADER_PREFETCH"
det.file_pattern = "bliss/tests/images/chart_2.edf"
det.nb_prefetched_frames = 1
ctl.image_roi =  [160, 60, 120, 200]

triggers InvalidValue: Roi out of limits max_roi=<0,0>-<0x0>, roi=<160,60>-<120x200>