vfio: Allow hotplug of containers onto existing guest IOMMU mappings

At present the memory listener used by vfio to keep host IOMMU mappings
in sync with the guest memory image assumes that if a guest IOMMU
appears, then it has no existing mappings.

This may not be true if a VFIO device is hotplugged onto a guest bus
which didn't previously include a VFIO device, and which has existing
guest IOMMU mappings.

Therefore, use the memory_region_register_iommu_notifier_replay()
function in order to fix this case, replaying existing guest IOMMU
mappings, bringing the host IOMMU into sync with the guest IOMMU.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
