qmp: fix handling of cmd with equal mark in qmp-shell

    qmp-shell splits the argument and value of input command
	by equal mark("="). But there are commands whose values
	include equal mark themselves, and the json built by
	qmp-shell will not correct. For example, when using NBD as
	the target of block-backup command, the input
	"block-backup target=nbd+unix:///drive0?socket=/tmp/nbd.sock"
	will fail, because the json built will be as follows:

    {
		"execute":"block-backup",
		"arguments":{"target":"nbd+unix:///drive0?socket"

}
	}

    Fix it by joining the sections split by equal mark excluding the
	first section in __build_cmd function when the length of sections
	is larger than two.

Signed-off-by: zhangleiqiang <zhangleiqiang@huawei.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>