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.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #814 from yumingfei/fix_virsh_module

virttest.virsh: Fix error of referenced before defined.