migration: make writes blocking

Move all the writes to the migration_thread, and make writings
blocking.  Notice that are still using the iothread for everything
that we do.

Signed-off-by: Juan Quintela <quintela@redhat.com>

migration-unix.c

@@ -60,6 +60,7 @@ static void unix_wait_for_connect(int fd, void *opaque)
     } else {
         DPRINTF("migrate connect success\n");
         s->fd = fd;
+        socket_set_block(s->fd);
         migrate_fd_connect(s);
     }
 }