x86: intel-iommu: add vt-d init test

Adding fundamental init test for Intel IOMMU. This includes basic
initialization of Intel IOMMU device, like DMAR (DMA Remapping),
IR (Interrupt Remapping), QI (Queue Invalidation), etc.

Further tests can use vtd_init() to initialize Intel IOMMU environment.

x86/unittests is updated to add this test.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>