This patch splits the xhci_xfer_data function into three.
The xhci_xfer_data function used to do does two things:

  (1) copy transfer data between guest memory and a temporary buffer.
  (2) report transfer results to the guest using events.

Now we three functions to handle this:

  (1) xhci_xfer_map creates a scatter list for the transfer and
      uses that (instead of the temporary buffer) to build a
      USBPacket.
  (2) xhci_xfer_unmap undoes the mapping.
  (3) xhci_xfer_report sends out events.

The patch also fixes reporting of transaction errors which must be
reported unconditinally, not only in case the guest asks for it
using the ISP flag.

[ v2: fix warning ]
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>