kvm-tools: write the sandbox command exactly as expect

sandbox.sh is a shell script, it will translate its meta
chars to anather things.

example(before this patch applied):
./lkvm sandbox -- echo 'success; please try again!'
it fails with:
/virt/sandbox.sh: line 3: please: command not found

Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>