memory: prepare AddressSpace for exporting

AddressSpace contains a member, current_map, of type FlatView.  Since we
want to limit the leakage of internal types to public headers, switch to
a pointer to a FlatView.  There is no performance impact as this isn't used
during lookups, only address space reconfigurations.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
