Avoid wild securityManager pointer in tests

For some reason we are not setting the driver with memset() to zeros.
But since commit 74abc3deac
driver->securityManager is being accessed and qemuagenttest started
crashing due to that.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

tests/testutilsqemu.c

@@ -558,6 +558,8 @@ int qemuTestDriverInit(virQEMUDriver *driver)
     if (virMutexInit(&driver->lock) < 0)
         return -1;
 
+    driver->securityManager = NULL;
+
     driver->config = virQEMUDriverConfigNew(false);
     if (!driver->config)
         goto error;