docs: Document acpi nodeset in hostdev

docs: Document acpi nodeset in hostdev

Add documentation for the new <acpi nodeset="..."> element in hostdev,
which allows associating devices with ACPI Generic Initiator objects in
QEMU.

A typical use case is NVIDIA Multi-Instance GPU (MIG), where a physical
GPU is partitioned into multiple isolated instances, each tied to one or
more virtual NUMA nodes. The documentation includes an example showing
how to configure <numa> cells together with a MIG device.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>