intel_iommu: add "caching-mode" option

This capability asks the guest to invalidate cache before each map operation.
We can use this invalidation to trap map operations in the hypervisor.

Signed-off-by: Aviv Ben-David <bd.aviv@gmail.com>
[peterx: using "caching-mode" instead of "cache-mode" to align with spec]
[peterx: re-write the subject to make it short and clear]
Reviewed-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Aviv Ben-David <bd.aviv@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>