background: refer to orignal plan posted by Marcelo Tosatti, http://lists.gnu.org/archive/html/qemu-devel/2012-06/msg04315.html
prev version: https://lists.gnu.org/archive/html/qemu-devel/2012-07/msg03312.html changes v1->v2 --introduce atomic ops --introduce ref cnt for MemoryRegion --make memory's flat view and radix tree toward rcu style.
