Merge remote-tracking branch 'remotes/kraxel/tags/pull-usb-20160622-2' into staging

usb: add hotplug support for usb-bot and usb-uas.

# gpg: Signature made Wed 22 Jun 2016 12:45:46 BST
# gpg:                using RSA key 0x4CB6D8EED3E87138
# 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/pull-usb-20160622-2:
  usb-uas: hotplug support
  usb-bot: hotplug support
  usb: Add QOM property "attached".
  usb: make USBDevice->attached bool
  usb-storage: qcow2 encryption support is finally gone, zap dead code

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