usb/redir: avoid dynamic stack allocation (CVE-2021-3527)

Use autofree heap allocation instead.

Fixes: 4f4321c1 ("usb: use iovecs in USBPacket")
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210503132915.2335822-3-kraxel@redhat.com>