Merge remote-tracking branch 'remotes/kraxel/tags/fixes-20200721-pull-request' into staging

fixes for xhci and modular builds.

# gpg: Signature made Tue 21 Jul 2020 15:02:33 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>" [full]
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>" [full]
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>" [full]
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/fixes-20200721-pull-request:
  module: ignore NULL type
  qxl: fix modular builds with dtrace
  xhci: fix valid.max_access_size to access address registers

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