kvm-tools: only one rootfs directory atmost

It makes no sense when we use several rootfs directories. Only the last
one works.

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