plugin: add qemu_plugin_outs helper

Having the plugins grab stdout and spew stuff there is a bit ugly and
certainly makes the tests look ugly. Provide a hook back into QEMU
which can be redirected as needed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Aaron Lindsay <aaron@os.amperecomputing.com>

include/qemu/qemu-plugin.h

@@ -384,4 +384,10 @@ int qemu_plugin_n_vcpus(void);
 /* returns -1 in user-mode */
 int qemu_plugin_n_max_vcpus(void);
 
+/**
+ * qemu_plugin_outs() - output string via QEMU's logging system
+ * @string: a string
+ */
+void qemu_plugin_outs(const char *string);
+
 #endif /* QEMU_PLUGIN_API_H */