xend: Fix use of set() for Python 2.3.

From: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

tools/python/xen/util/pci.py

@@ -19,6 +19,12 @@ from xen.xend import sxp
 from xen.xend.XendConstants import AUTO_PHP_SLOT
 from xen.xend.XendSXPDev import dev_dict_to_sxp
 
+# for 2.3 compatibility
+try:
+    set()
+except NameError:
+    from sets import Set as set
+
 PROC_PCI_PATH = '/proc/bus/pci/devices'
 PROC_PCI_NUM_RESOURCES = 7