AMD/IOMMU: provide function backing XENMEM_reserved_device_memory_map

AMD/IOMMU: provide function backing XENMEM_reserved_device_memory_map

Just like for VT-d, exclusion / unity map ranges would better be
reflected in e.g. the guest's E820 map. The reporting infrastructure
was put in place still pretty tailored to VT-d's needs; extend
get_reserved_device_memory() to allow vendor specific code to probe
whether a particular (seg,bus,dev,func) tuple would get its data
actually recorded. I admit the de-duplication of entries is quite
limited for now, but considering our trouble to find a system
surfacing _any_ IVMD this is likely not a critical issue for this
initial implementation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>