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/linuxbridge b/lib/quantum_plugins/linuxbridge
index e8ba68c..6d5d4e0 100644
--- a/lib/quantum_plugins/linuxbridge
+++ b/lib/quantum_plugins/linuxbridge
@@ -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() {
@@ -76,4 +76,4 @@
 }
 
 # Restore xtrace
-$XTRACE
+$MY_XTRACE