util: vhost user: support for bootindex

util: vhost user: support for bootindex

Problem Description:
When we set boot order for a vhost-user network interface, we found the boot index
doesn't work.

Cause of the Problem:
In the function qemuBuildVhostuserCommandLine(), it forcely set the arg bootindex of
function qemuBuildNicDevStr() to 0. Thus, the bootindex parameter got missing.

Solution:
Trans the arg bootindex down.

Signed-off-by: Gao Haifeng <gaohaifeng.gao@huawei.com>
Signed-off-by: Zhang Bo <oscar.zhangbo@huawei.com>