nvdimm: convert nvdimm_mr into a pointer

This way we can easily check if the region has already been inititalized
without having to rely on the size of an uninitialized region being 0.

Free the region in nvdimm_finalize() and not in unrealize() as we will
allow to create the region before realization in following patches.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180619134141.29478-11-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>