Merge remote-tracking branch 'remotes/dgilbert/tags/pull-migration-20180410a' into staging

Migration reversion pull for 2.12

One to revert after we decided it needs some more thinking.

# gpg: Signature made Tue 10 Apr 2018 16:02:17 BST
# gpg:                using RSA key 0516331EBC5BFDE7
# gpg: Good signature from "Dr. David Alan Gilbert (RH2) <dgilbert@redhat.com>"
# Primary key fingerprint: 45F5 C71B 4A0C B7FB 977A  9FA9 0516 331E BC5B FDE7

* remotes/dgilbert/tags/pull-migration-20180410a:
  Revert "migration: Don't activate block devices if using -S"

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>