Merge remote-tracking branch 'remotes/kraxel/tags/pull-usb-20160511-1' into staging

usb: misc fixes

# gpg: Signature made Wed 11 May 2016 12:18:25 BST 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-usb-20160511-1:
  usb: Support compilation without poll.h
  usb-mtp: fix usb_mtp_get_device_info so that libmtp on the guest doesn't complain
  usb:xhci: no DMA on HC reset

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