ringtest: commonize implementation of poll_avail/poll_used

Provide new primitives used_empty/avail_empty and
build poll_avail/poll_used on top of it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/ringtest/noring.c

@@ -24,8 +24,9 @@ void *get_buf(unsigned *lenp, void **bufp)
 	return "Buffer";
 }
 
-void poll_used(void)
+bool used_empty()
 {
+	return false;
 }
 
 void disable_call()
@@ -54,8 +55,9 @@ bool enable_kick()
 	assert(0);
 }
 
-void poll_avail(void)
+bool avail_empty()
 {
+	return false;
 }
 
 bool use_buf(unsigned *lenp, void **bufp)