iommu/vt-d: Allocate dynamic domain id for virtual domains only

Check the same domain id is allocated for si_domain on each IOMMU,
otherwise the IOTLB flush for si_domain will fail.

Now the rules to allocate and manage domain id are:
1) For normal and static identity domains, domain id is allocated
   when creating domain structure. And this id will be written into
   context entry.
2) For virtual machine domain, a virtual id is allocated when creating
   domain. And when binding virtual machine domain to an iommu, a real
   domain id is allocated on demand and this domain id will be written
   into context entry. So domain->id for virtual machine domain may be
   different from the domain id written into context entry(used by
   hardware).

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>