iommu/virtio: Build virtio-iommu as module

Now that the infrastructure changes are in place, enable virtio-iommu to
be built as a module. Remove the redundant pci_request_acs() call, since
it's not exported but is already invoked during DMA setup.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>