Merge remote-tracking branch 'remotes/dgilbert/tags/pull-migration-20180329a' into...

Migration pull (small fixes)

A pair of two small fixes for 2.12.

# gpg: Signature made Thu 29 Mar 2018 14:55:17 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-20180329a:
  migration: Don't activate block devices if using -S
  migration: fix pfd leak

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