memory/iommu: Add get_attr()

This adds get_attr() to IOMMUMemoryRegionClass, like
iommu_ops::domain_get_attr in the Linux kernel.

This defines the first attribute - IOMMU_ATTR_SPAPR_TCE_FD - which
will be used between the pSeries machine and VFIO-PCI.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>