VT-d: size qinval queue dynamically

VT-d: size qinval queue dynamically

With the present synchronous model, we need two slots for every
operation (the operation itself and a wait descriptor).  There can be
one such pair of requests pending per CPU. To ensure that under all
normal circumstances a slot is always available when one is requested,
size the queue ring according to the number of present CPUs.

This is part of XSA-373 / CVE-2021-28692.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
(cherry picked from commit cbfa62bb140c8f10a0ae96db3ce06e22b3b9d302)