Fix the DISPLAY configration that breaks vmx domain creation with SDL

on vncserver or remote X. Changeset 7547 added a test of
opts.vals.display when getenv("DISPLAY"); because there is a default
value:localhost:0, that causes the test always to be true.

Signed-off-by: Xiaofeng Ling <xiaofeng.ling@intel.com>

tools/python/xen/xm/create.py

@@ -376,7 +376,7 @@ gopts.var('sdl', val='',
           use="""Should the device model use SDL?""")
 
 gopts.var('display', val='DISPLAY',
-          fn=set_value, default='localhost:0',
+          fn=set_value, default=None,
           use="X11 display to use")