Merge remote-tracking branch 'remotes/kraxel/tags/pull-fw-cfg-20160226-1' into staging

fw_cfg: unbreak migration compatibility for 2.4 and earlier machines

# gpg: Signature made Fri 26 Feb 2016 09:45:50 GMT using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-fw-cfg-20160226-1:
  fw_cfg: unbreak migration compatibility for 2.4 and earlier machines

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