AMD/IOMMU: size command buffer dynamically

AMD/IOMMU: size command buffer dynamically

With the present synchronous model, we need two slots for every
operation (the operation itself and a wait command).  There can be one
such pair of commands pending per CPU. To ensure that under all normal
circumstances a slot is always available when one is requested, size the
command 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 df242851ddc93ac0b0a3a20ecab34acc29e3b36b)