intel_iommu: Add support for translation for devices behind bridges

- Use a hash table indexed on bus pointers to store information about buses
  instead of using the bus numbers.
  Bus pointers are stored in a new VTDBus struct together with the vector
  of device address space pointers indexed by devfn.
- The bus number is still used for lookup for selective SID based invalidate,
  in which case the bus number is lazily resolved from the bus hash table and
  cached in a separate index.

Signed-off-by: Knut Omang <knut.omang@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>