Revert "virtio: unbreak virtio-pci with IOMMU after caching ring translations"

This reverts commit
96a8821d. Previous patch is a better
solution which does not require a strict order between virtio and IOMMU.

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>