docs: Add '-device intel-iommu' entry

docs: Add '-device intel-iommu' entry

The parameters of intel-iommu device are non-trivial to understand.  Add an
entry for it so that people can reference to it when using.

There're actually a few more options there, but I hide them explicitly because
they shouldn't be used by normal QEMU users.

Cc: Chao Yang <chayang@redhat.com>
Cc: Lei Yang <leiyang@redhat.com>
Cc: Jing Zhao <jinzhao@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210707154114.197580-1-peterx@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>