Handle nested xtrace states (Quantum edition)

The lib/quantum_* directories include plugin files that also
contains the $XTRACE bits at entry and exit.  The nested XTRACE handling overwrote
the value from lib/quantum.  So...make the nested files use their own
XTRACE variables.

Change-Id: Ib5e643371666b21402eef2ec58bfb1dfb7e1ccc4
diff --git a/lib/quantum_plugins/ovs_base b/lib/quantum_plugins/ovs_base
index d9f6fd0..4c33473 100644
--- a/lib/quantum_plugins/ovs_base
+++ b/lib/quantum_plugins/ovs_base
@@ -2,7 +2,7 @@
 # -------------------------------------
 
 # Save trace setting
-XTRACE=$(set +o | grep xtrace)
+MY_XTRACE=$(set +o | grep xtrace)
 set +o xtrace
 
 function is_quantum_ovs_base_plugin() {
@@ -46,4 +46,4 @@
 }
 
 # Restore xtrace
-$XTRACE
+$MY_XTRACE