Merge remote-tracking branch 'remotes/philmd-gitlab/tags/fw_cfg-20200918' into staging

fw_cfg patches

Fixes a bug in the recently introduced fw_cfg_add_from_generator().

CI jobs results:
. https://gitlab.com/philmd/qemu/-/pipelines/191714875
. https://travis-ci.org/github/philmd/qemu/builds/728288784
. https://app.shippable.com/github/philmd/qemu/runs/869/1/console

# gpg: Signature made Fri 18 Sep 2020 16:20:56 BST
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd-gitlab/tags/fw_cfg-20200918:
  hw/nvram/fw_cfg: fix FWCfgDataGeneratorClass::get_data() consumption

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