Add xz-support

[b9AcE]

As several of the distros used with wsldl provide their rootfs.tar or equivalent in the significantly better compressed xz format instead of, or in addition to, the traditional gzip compression, it would be convenient if wsldl also recognized and was able to import a "rootfs.tar.xz" file without the users having to first decompress, recompress and then have wsldl decompress it again during the install-importing anyway.

Maybe even also add support for uncompressed plain "rootfs.tar" files as well, because... it only takes checking for the file and in that case skipping the decompression, so why not?

[Biswa96]

This is WSL's issue. See microsoft/WSL#6056

[yuk7]

Supporting xz requires an external library like xz util and is cumbersome as it requires more processing.

[yuk7]

Even on the wsldl side, XZ support is not possible due to license reasons.
I like GPL, but I'm not thinking about making wsldl a GPL license.

[yuk7]

I don't like beer, so I prefer free speech to free beer.

[yuk7]

added tar file support in 96e1125.
but xz is not yet.