get_system_memory() provides the root of the memory hierarchy.

This interface is intended to be private between memory.c and exec.c.
If this file is included elsewhere, it should be regarded as a bug (or
TODO item).  However, it will be temporarily needed for the conversion
to hierarchical memory routing.
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>