io: Remove redundant read/write_coroutine assignments

qio_channel_yield() now updates ioc->read_write/coroutine and calls
qio_channel_set_aio_fd_handlers(), so the code in the handlers has
become redundant and can be removed.

This does not make a difference in intermediate states because
aio_co_wake() really enters the coroutine immediately here: These
handlers are never run in coroutine context, and we're in the right
AioContext because qio_channel_attach_aio_context() asserts that the
handlers are inactive.

To make these conditions more obvious, assert the right AioContext.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

io/channel.c

@@ -400,15 +400,14 @@ off_t qio_channel_io_seek(QIOChannel *ioc,
 }
 
 
-static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc);
-
 static void qio_channel_restart_read(void *opaque)
 {
     QIOChannel *ioc = opaque;
     Coroutine *co = ioc->read_coroutine;
 
-    ioc->read_coroutine = NULL;
-    qio_channel_set_aio_fd_handlers(ioc);
+    /* Assert that aio_co_wake() reenters the coroutine directly */
+    assert(qemu_get_current_aio_context() ==
+           qemu_coroutine_get_aio_context(co));
     aio_co_wake(co);
 }
 
@@ -417,8 +416,9 @@ static void qio_channel_restart_write(void *opaque)
     QIOChannel *ioc = opaque;
     Coroutine *co = ioc->write_coroutine;
 
-    ioc->write_coroutine = NULL;
-    qio_channel_set_aio_fd_handlers(ioc);
+    /* Assert that aio_co_wake() reenters the coroutine directly */
+    assert(qemu_get_current_aio_context() ==
+           qemu_coroutine_get_aio_context(co));
     aio_co_wake(co);
 }