TYPE_AMD_IOMMU_PCI is user-creatable but not well described.
Implement its class_init() handler to add it to the 'Misc
devices' category, and add a description.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210926175648.1649075-4-f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>