XZ threads are very bad about memory, so use only two threads.
In koji #380 we setup -T0
That will use all available compute resources.
It also has the potential to exhaust system memnory, and cause errors.
This PR seeks a middle ground with 2 XZ threads similar to createrepo_c.
https://github.com/rpm-software-management/createrepo_c/blob/master/src/compression_wrapper.c#L490
Signed-off-by: jdisnard <jdisnard@redhat.com>