memory: MemoryRegion: factor out subregion add functionality

Split off the core looping code that actually adds subregions into
it's own fn. This prepares support for Memory Region qomification
where setting the MR address or parent via QOM will back onto this more
minimal function.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
[Rename new function. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
