memory: Use memory_region_name for name access

Despite being local to memory.c, use the helper function. This prepares
support for fully QOMifiying the name field of MR (which will remove
this state from MR completely).

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