uhci: Use an intermediate buffer for usb packet data

uhci: Use an intermediate buffer for usb packet data

Due to various unfortunate reasons we cannot reliable detect a guest
cancelling a packet as soon as it happens, instead we detect cancels
with some delay.

When packets are handled async, and we directly pass the guest memory for
the packet to the usb-device as iovec, this means that the usb-device can
write to guest-memory which the guest has already re-used for other purposes
-> not good!

This patch fixes this by adding an intermediate buffer and writing back not
only the result, but also the data, of async completed packets when scanning
the schedule.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>