usb: bugfixes, doc update

# gpg: Signature made Fri 12 May 2017 01:20:29 PM 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

* kraxel/tags/pull-usb-20170512-1:
  hw/usb/dev-serial: Do not try to set vendorid or productid properties
  xhci: relax link check
  usb-hub: clear PORT_STAT_SUSPEND on wakeup
  xhci: fix logging
  usb-redir: fix stack overflow in usbredir_log_data
  qemu-doc: Update to use the new way of attaching USB devices
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>