Generate gdbstub-xml.c only when needed

First user of new config-devices.mak

Patchworks-ID: 35198
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/default-configs/ppcemb-softmmu.mak b/default-configs/ppcemb-softmmu.mak
index 951bfa0..bff13e6 100644
--- a/default-configs/ppcemb-softmmu.mak
+++ b/default-configs/ppcemb-softmmu.mak
@@ -1 +1,3 @@
 # Default configuration for ppcemb-softmmu
+
+CONFIG_GDBSTUB_XML=y