scripts/qemustart: more portable array operation

scripts/qemustart: more portable array operation

The following commands output 1,2,1,1

    cmd0='a=("${a[@]}" 'a'); echo "${#a}"'
    cmd1='a+=('a'); echo "${#a}"'
    bash -c "$cmd0"; zsh -c "$cmd0"
    bash -c "$cmd1"; zsh -c "$cmd1"

The following outputs 0,1,0,0

    cmd2='f() { echo "$#"; }; f "${a[@]}"'
    cmd3="a=(); $cmd2"
    bash -c "$cmd2"; zsh -c "$cmd2"
    bash -c "$cmd3"; zsh -c "$cmd3"

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>