Add an API that allows a client to observe changes in the global
memory map:
 - region added (possibly with logging enabled)
 - region removed (possibly with logging enabled)
 - logging started on a region
 - logging stopped on a region
 - global logging started
 - global logging removed

This API will eventually replace cpu_register_physical_memory_client().
Signed-off-by: NAvi Kivity <avi@redhat.com>