vfio: Support VFIO_NOIOMMU with iommufd

Add a small amount of emulation to vfio_compat to accept the SET_IOMMU to
VFIO_NOIOMMU_IOMMU and have vfio just ignore iommufd if it is working on a
no-iommu enabled device.

Move the enable_unsafe_noiommu_mode module out of container.c into
vfio_main.c so that it is always available even if VFIO_CONTAINER=n.

This passes Alex's mini-test:

https://github.com/awilliam/tests/blob/master/vfio-noiommu-pci-device-open.c

Link: https://lore.kernel.org/r/0-v3-480cd64a16f7+1ad0-iommufd_noiommu_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>