Change compression or add an option for compress=zstd

[mk3d]

Hello,

I love the xenbackup-ng script from while and thanks for that!

Any chance to change the default compression or add an option to use the "new" zstandard compression?
As described here:
https://xcp-ng.org/blog/2018/12/19/zstd-compression-for-xcp-ng/

I already change the script to add compress=zstd in place and the performance is awesome! about x5 speed.

Many thanks in advance and best regards.

[rbicelli]

Hello! Could you make a Pull request? I surely accept it!