qemu_conf: properly set 'deprecation_behavior' default value

The comment for that option states that the default value is 'none' but
it was not set by the code. By default the value is NULL which results
into the following warning:

warning : qemuBuildCompatDeprecatedCommandLine:10393 : Unsupported deprecation behavior '(null)' for VM 'test'

Fixes: 7004504493
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_conf.c

@@ -295,6 +295,8 @@ virQEMUDriverConfigPtr virQEMUDriverConfigNew(bool privileged,
                              &cfg->nfirmwares) < 0)
         return NULL;
 
+    cfg->deprecationBehavior = g_strdup("none");
+
     return g_steal_pointer(&cfg);
 }