KVM: Remove ability to assign a device without iommu support

KVM: Remove ability to assign a device without iommu support

commit 423873736b78f549fbfa2f715f2e4de7e6c5e1e9 upstream.

This option has no users and it exposes a security hole that we
can allow devices to be assigned without iommu protection.  Make
KVM_DEV_ASSIGN_ENABLE_IOMMU a mandatory option.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
[PG: drop Documentation chunk; no api.txt file in .34 baseline]
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>