generic/108: Fix return value check from _get_scsi_debug_dev

generic/108: Fix return value check from _get_scsi_debug_dev

_get_scsi_debug_dev is supposed to return a "/dev/$device".
However, in case the scsi device is not mapped to a disk, hence
/dev/sd* doesn't exist, then get_scsi_debug_dev would return only
the "/dev/" string. In generic/108 we check whether return value is
"" and only then consider it a failure. This behavior allows the
test to erroneously consider _get_scsi_debug_dev succeeded even if
it returned a malformed string. Fix this by correctly checking
whether the return value is "/dev/"

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>