selftests/bpf: Support local rootfs image for vmtest

selftests/bpf: Support local rootfs image for vmtest

Support vmtest to use local rootfs image generated by [0] that is
consistent with BPF CI. Now we can specify the local rootfs image
through the `-l` parameter like as follows:

  vmtest.sh -l ./libbpf-vmtest-rootfs-2024.08.22-noble-amd64.tar.zst -- ./test_progs

Meanwhile, some descriptions have been flushed.

Link: https://github.com/libbpf/ci/blob/main/rootfs/mkrootfs_debian.sh
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-6-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>