In the large message case, the sender issues a PtlMEAppend() in
order to generate events when the receiver issues a PtlGet(). This
commit moves the PtlMEAppend() from mca_btl_portals4_prepare_src()
to mca_btl_portals4_register_mem() which is the way it's done in
BTL 3.0.
bca2522d8b