A refactoring step to prepare for the problem
exposed by the test-listen test in the previous commit.

Simplify and reorganize the IPv6 specific extra
measures and move it out of the for loop to increase
code readability. No semantic changes.

Signed-off-by: Knut Omang <knut.omang@oracle.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>