pc-dimm: do not allow setting an in-use memdev using the same memdev backend more than once will cause assertion at MemoryRegion mapping time because it's already mapped. Prevent it by checking that the associated MemoryRegion is not mapped. Signed-off-by: Igor Mammedov <imammedo@redhat.com> Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> MST: tweak commit log