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-fd.c

@@ -75,7 +75,6 @@ void fd_start_outgoing_migration(MigrationState *s, const char *fdname, Error **
         return;
     }
 
-    fcntl(s->fd, F_SETFL, O_NONBLOCK);
     s->get_error = fd_errno;
     s->write = fd_write;
     s->close = fd_close;