amd iommu: Add iommu emulation for hvm guest

amd iommu: Add iommu emulation for hvm guest

ATS device driver that support PASID [1] and PRI [2] capabilites needs
to work with iommu driver in guest OS. We have to expose iommu
functionality to HVM guest, if we want assign ATS device to it. A new
hypervisor mmio handler is added to intercept iommu mmio accesses from
guest.

Signed-off-by: Wei Wang <wei.wang2@amd.com>
[1] http://www.pcisig.com/specifications/pciexpress/specifications/ECN-PASID-ATS-2011-03-31.pdf
[2] http://www.pcisig.com/members/downloads/specifications/iov/ats_r1.1_26Jan09.pdf
Committed-by: Jan Beulich <jbeulich@suse.com>