virsh: Init virsh_exec with default value if it is None.

Without this patch, we will get a virsh cmd like "None -c uri start vm".

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #1698 from yangdongsheng/start

virsh: Init virsh_exec with default value if it is None.