xm: Set device_model option value default to None.
If device_model is set to '', xend will fail to execute the device model. 
If None, xend will detect it and set a sensible qemu-dm path instead.

Signed-off-by: Yosuke Iwamatsu <y-iwamatsu@ab.jp.nec.com>

diff -r b320cfe1f10f tools/python/xen/xm/create.py
--- a/tools/python/xen/xm/create.py	Thu Jun 05 13:04:07 2008 +0100
+++ b/tools/python/xen/xm/create.py	Mon Jun 09 15:07:36 2008 +0900
@@ -424,7 +424,7 @@ gopts.var('nfs_root', val="PATH",
           use="Set the path of the root NFS directory.")
 
 gopts.var('device_model', val='FILE',
-          fn=set_value, default='',
+          fn=set_value, default=None,
           use="Path to device model program.")
 
 gopts.var('fda', val='FILE',