migration: split migration_incoming_co

Originally, migration_incoming_co was introduced by
25d0c16f62
   "migration: Switch to COLO process after finishing loadvm"
to be able to enter from COLO code to one specific yield point, added
by 25d0c16f62.

Later in 923709896b
 "migration: poll the cm event for destination qemu"
we reused this variable to wake the migration incoming coroutine from
RDMA code.

That was doubtful idea. Entering coroutines is a very fragile thing:
you should be absolutely sure which yield point you are going to enter.

I don't know how much is it safe to enter during qemu_loadvm_state()
which I think what RDMA want to do. But for sure RDMA shouldn't enter
the special COLO-related yield-point. As well, COLO code doesn't want
to enter during qemu_loadvm_state(), it want to enter it's own specific
yield-point.

As well, when in 8e48ac9586
 "COLO: Add block replication into colo process" we added
bdrv_invalidate_cache_all() call (now it's called activate_all())
it became possible to enter the migration incoming coroutine during
that call which is wrong too.

So, let't make these things separate and disjoint: loadvm_co for RDMA,
non-NULL during qemu_loadvm_state(), and colo_incoming_co for COLO,
non-NULL only around specific yield.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230515130640.46035-3-vsementsov@yandex-team.ru>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/rdma.c

@@ -3342,9 +3342,8 @@ static void rdma_cm_poll_handler(void *opaque)
             }
         }
         rdma_ack_cm_event(cm_event);
-
-        if (mis->migration_incoming_co) {
-            qemu_coroutine_enter(mis->migration_incoming_co);
+        if (mis->loadvm_co) {
+            qemu_coroutine_enter(mis->loadvm_co);
         }
         return;
     }