memory: Lazy init name from QOM name as needed

To support name retrieval of MemoryRegions that were created
dynamically (that is, not via memory_region_init and friends). We
cache the name in MemoryRegion's state as
object_get_canonical_path_component mallocs the returned value
so it's not suitable for direct return to callers. Memory already
frees the name field, so this will be garbage collected along with
the MR object.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>