xen/usb: harden xen_hcd against malicious backends

Make sure a malicious backend can't cause any harm other than wrong
I/O data.

Missing are verification of the request id in a response, sanitizing
the reported actual I/O length, and protection against interrupt storms
from the backend.

Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220311103509.12908-1-jgross@suse.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>