tools: Use os.system instead of exec to start xentop.

This makes no difference to the visible behaviour of "xm top" but
avoids the whole of xm from quitting if you're in "xm shell".  This
provides the more intuitive behaviour of returning to the xm shell
interface after leaving xm top.

Signed-off-by: Mark Williamson <mark.williamson@cl.cam.ac.uk>

tools/python/xen/xm/main.py

@@ -1833,7 +1833,7 @@ def xm_debug_keys(args):
 def xm_top(args):
     arg_check(args, "top", 0)
 
-    os.execvp('xentop', ['xentop'])
+    os.system('xentop')
 
 def xm_dmesg(args):
     arg_check(args, "dmesg", 0, 1)