virt: Improves list tests function.

Allow use new params build-in variable which maps file to vartiant name.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
virt: Adds possibility to filter tests based on netconf.

Some of tests not works with nettype=user, but there wasn't
good way how to filter the tests.

Signed-off-by: Jiří Župka <jzupka@redhat.com>