classes/testimage: if start fails, don't try to stop

If we couldn't start the target, it doesn't make sense to try and stop
it here since logically it shouldn't now be in any kind of "started"
state. (It's the start function's job to clean up after itself if it
fails - to that end, fix up the QemuTarget class so that it does.)

(From OE-Core rev: 819ebddae6b78120e5e082423793ff988419b5c4)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/testimage.bbclass

@@ -209,8 +209,8 @@ def testimage_main(d):
 
     target.deploy()
 
+    target.start()
     try:
-        target.start()
         if export:
             exportTests(d,tc)
         else: