autotest/virt-test — pull request #814 by yumingfei (2013-09-03 10:06:57+00:00)
Title: virttest.virsh: Fix error of referenced before defined. 

a += b equals to a = a + b.
So for self.virsh_exec += " -c xxx" in VirshSession.
It will cause problem of referenced before defined.
Now define self.virsh_exec before this line.