hw/usb/xen-usb.c: Pass struct usbback_req* to usbback_packet_complete()

hw/usb/xen-usb.c: Pass struct usbback_req* to usbback_packet_complete()

The function usbback_packet_complete() currently takes a USBPacket*,
which must be a pointer to the packet field within a struct
usbback_req; the function uses container_of() to get the struct
usbback_req* given the USBPacket*.

This is unnecessarily confusing (and in particular it confuses the
Coverity Scan analysis, resulting in the false positive CID 1421919
where it thinks that we write off the end of the structure). Since
both callsites already have the pointer to the struct usbback_req,
just pass that in directly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200323164318.26567-1-peter.maydell@linaro.org>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>