iommu/vt-d: Use correct domain id to flush virtual machine domains

For virtual machine domains, domain->id is a virtual id, and the real
domain id written into context entry is dynamically allocated.
So use the real domain id instead of domain->id when flushing iotlbs
for virtual machine domains.

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