virtio-devices: iommu: allow limiting maximum address width in bits

Currently, Cloud Hypervisor does not set a VIRTIO_IOMMU_F_INPUT_RANGE
feature bit for the VirtIO IOMMU device, which, according to spec[1],
means that the guest may use the whole 64-bit address space is for
IOMMU purposes:

>If the feature is not offered, virtual mappings span over the whole
>64-bit address space (start = 0, end = 0xffffffff ffffffff)

As far as I am aware, there are currently no host platforms on
the market capable of addressing the whole 64-bit address space.

For example, I am currently working with a host platform that reports
39-bit address space for IOMMU purposes:

>DMAR: Host address width 39

When running a VFIO pass-through guest on such a platform, NVIDIA
driver in guest gets DMA mapping failures when working with large data,
and this results in Cloud Hypervisor exiting with the following error:

>cloud-hypervisor: 1501.220535s: <__iommu>
>ERROR:virtio-devices/src/thread_helper.rs:53 -- Error running worker:
>HandleEvent(Failed to process request queue : ExternalMapping(Custom
>{ kind: Other, error: "failed to map memory for VFIO container, iova
>0x7fff00000000, gpa 0x24ce25000, size 0x1000: IommuDmaMap(Error(22))"
>}))

Passing "--platform iommu_address_width=39" to Cloud Hypervisor built
with this change fixes this.

[1]: https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/
virtio-v1.3-csd01.html#x1-5420006

Signed-off-by: Nikolay Edigaryev <edigaryev@gmail.com>

fuzz/fuzz_targets/iommu.rs

@@ -66,6 +66,7 @@ fuzz_target!(|bytes: &[u8]| -> Corpus {
         SeccompAction::Allow,
         EventFd::new(EFD_NONBLOCK).unwrap(),
         ((MEM_SIZE - IOVA_SPACE_SIZE) as u64, (MEM_SIZE - 1) as u64),
+        64,
         None,
     )
     .unwrap();