Merge remote-tracking branch 'remotes/kraxel/tags/pull-fw-cfg-20151217-1' into staging

fw_cfg: doc updates, various optimizations.

# gpg: Signature made Thu 17 Dec 2015 08:59:32 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-20151217-1:
  fw_cfg: replace ioport data read with generic method
  fw_cfg: add generic non-DMA read method
  fw_cfg: avoid calculating invalid current entry pointer
  fw_cfg: remove offset argument from callback prototype
  fw_cfg: amend callback behavior spec to once per select
  fw_cfg: move internal function call docs to header file

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