memory: change memory registration to rebuild the memory map on each change

Instead of incrementally building the memory map, rebuild it every time.
This allows later simplification, since the code need not consider overlaying
a previous mapping.  It is also RCU friendly.

With large memory guests this can get expensive, since the operation is
O(mem size), but this will be optimized later.

As a side effect subpage and L2 leaks are fixed here.

Signed-off-by: Avi Kivity <avi@redhat.com>