block: Add coroutine_fn to bdrv_check_co_entry

bdrv_check_co_entry calls bdrv_co_check, which is a coroutine function.
Thus, it also needs to be marked as a coroutine.

Signed-off-by: Nikita Alekseev <n.alekseev2104@gmail.com>
Message-id: 20190401093051.16488-1-n.alekseev2104@gmail.com
Message-Id: <20190401093051.16488-1-n.alekseev2104@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>