coroutine-lock: qemu_co_queue_next is a coroutine-only qemu_co_enter_next

qemu_co_queue_next is basically the same as qemu_co_enter_next but
without a QemuLockable argument.  That's perfectly fine, but only
as long as the function is marked coroutine_fn.  If used outside
coroutine context, qemu_co_queue_wait will attempt to take the lock
and that is just broken: if you are calling qemu_co_queue_next outside
coroutine context, the lock is going to be a QemuMutex which cannot be
taken twice by the same thread.

The patch adds the marker and reimplements qemu_co_queue_next in terms of
qemu_co_enter_next_impl, to remove duplicated code and to clarify that the
latter also works in coroutine context.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220427130830.150180-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/coroutine.h

@@ -208,11 +208,12 @@ void qemu_co_queue_init(CoQueue *queue);
 void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
 
 /**
- * Removes the next coroutine from the CoQueue, and wake it up.
+ * Removes the next coroutine from the CoQueue, and queue it to run after
+ * the currently-running coroutine yields.
  * Returns true if a coroutine was removed, false if the queue is empty.
- * OK to run from coroutine and non-coroutine context.
+ * Used from coroutine context, use qemu_co_enter_next outside.
  */
-bool qemu_co_queue_next(CoQueue *queue);
+bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
 
 /**
  * Empties the CoQueue; all coroutines are woken up.