AMD IOMMU: remove currently non-functioning guest iommu feature

AMD IOMMU: remove currently non-functioning guest iommu feature

The guest IOMMU feature is currently not functioning. However,
the current guest_iommu_init() is causing issue when it tries to
register mmio handler because the it is currently called by the
following code path:

  arch/x86/domain.c: arch_domain_create()
    ]- drivers/passthrough/iommu.c: iommu_domain_init()
      |- drivers/passthrough/amd/pci_amd_iommu.c: amd_iommu_domain_init();
        |- drivers/passthrough/amd/iommu_guest.c: guest_iommu_init()

At this point, the hvm_domain_initialised() has not been called.
So register_mmio_handler() in guest_iommu_init() silently fails.

This patch removes the guest IOMMU feature for now until we can properly
support it.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>