vfio: Add vfio_container_ioctl()

While most operations with VFIO IOMMU driver are generic and used inside
vfio.c, there are still some operations which only specific VFIO IOMMU
drivers implement. The first example of it will be reading a DMA window
start from the host.

This adds a helper which passes an ioctl request to the container's fd.

The helper will check if @req is known. For this, stub is added. This return
-1 on any requests for now.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
