virtme-scripts: Add LSAN_OPTION

In preparation of turning mem leak detection on in CI, this patch adds
LSAN_OPTIONS to be able to pass the suppression file.

This *does not* turn the mem leak detection on yet, as we still
need some additional mem leak fixes.

This removes fast unwind set to zero as well, as that will add a massive
runtime penalty to some of our tests, and we'll no longer need it as
we've included the entire libfontconfig as a mem leak source in the
suppression file.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>