usb: add xen pvUSB backend, add num-ports check to ohci.

# gpg: Signature made Mon 23 May 2016 14:02:25 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-20160523-1:
  usb/ohci: Fix crash with when specifying too many num-ports
  xen: add pvUSB backend
  xen: write information about supported backends
  xen: introduce dummy system device

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