conf: Fix memory leak caused by missing VIR_FREE for video resolution.

Commit 72862797 introduced resolution settings for QEMU video drivers.
It includes a new structure inside video definition. So, the code needs
to clear pointer allocation for that structure into clear function
virDomainVideoDefClear(). This commit adds this missing VIR_FREE().
Reviewed-by: NJonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>

src/conf/domain_conf.c

@@ -2826,6 +2826,7 @@ virDomainVideoDefClear(virDomainVideoDefPtr def)
     if (def->accel)
         VIR_FREE(def->accel->rendernode);
     VIR_FREE(def->accel);
+    VIR_FREE(def->res);
     VIR_FREE(def->virtio);
     if (def->driver)
         VIR_FREE(def->driver->vhost_user_binary);