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>

plugins/qemu-plugins.symbols

@@ -36,4 +36,5 @@
   qemu_plugin_vcpu_for_each;
   qemu_plugin_n_vcpus;
   qemu_plugin_n_max_vcpus;
+  qemu_plugin_outs;
 };