Similarly to previous commits use real FDs so that we don't risk
collisions.
Note that for the test cases passing multiple tap and vhost FDs the
helpers which stabilize the output don't actually work, as the FDs are
concatenated. For now we'll not deal with this and simply leave them
censored by the 'XXXXXXX' string.
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>