exec: put memory map in AddressSpaceDispatch

exec: put memory map in AddressSpaceDispatch

After this patch, AddressSpaceDispatch holds a constistent tuple of
(phys_map, nodes, sections).  This will be important when updates
of the topology will run concurrently with reads.

cur_map is not used anymore except for freeing it at the end of the
topology update.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>