None of its callers checks for failure.  scsi_hot_add() can crash
because of that:

(qemu) drive_add 4 if=scsi,format=host_device,file=/dev/sg1
scsi-generic: scsi generic interface too old
Segmentation fault (core dumped)

Fix all callers, not just scsi_hot_add().
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>