dissect: use DISKSEQ when waiting for block devices

DISKSEQ is a reliable way to find out if we missed a uevent or not, as
it's monotonically increasing. If we parse an event with a smaller or
no sequence number, we know we need to wait longer. If we parse an
event with a greater sequence number, we know we missed it and the
device was reused.

src/nspawn/nspawn.c

@@ -5700,6 +5700,7 @@ static int run(int argc, char *argv[]) {
                                 arg_image,
                                 &arg_verity_settings,
                                 NULL,
+                                loop->diskseq,
                                 loop->uevent_seqnum_not_before,
                                 loop->timestamp_not_before,
                                 dissect_image_flags,