update_engine: fix verify for brillo major version

Currently since the major version is checked before the apply
payload (for brillo_update_payload verify), then it fails if there are
more or less than two partitions. This patch forces the major version to
whatever the target image is. This will not impact any production
procedure.

BUG=none
TEST=brillo_update_payload verify --payload=payload.delta --source_image=25-4k-gzip.img --target_image=61-4k-gzip.img

Change-Id: I0af9a56f9ad52c6b0670f7ace6c307d6071a1d7a
Reviewed-on: https://chromium-review.googlesource.com/629638


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>