Merge remote-tracking branch 'remotes/kraxel/tags/usb-20180618-pull-request' into staging

usb: revert ccid / qom patches.

# gpg: Signature made Mon 18 Jun 2018 10:21:11 BST
# gpg:                using RSA key 4CB6D8EED3E87138
# 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>"
# Primary key fingerprint: A032 8CFF B93A 17A7 9901  FE7D 4CB6 D8EE D3E8 7138

* remotes/kraxel/tags/usb-20180618-pull-request:
  Revert "bus: do not unref the added child bus on realize"
  Revert "usb: release the created buses"
  Revert "usb-ccid: fix bus leak"

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