tests: add IPv6-related unit tests
tests: add IPv6-related unit tests

Check produced libvirt XML, and QubesDB entries

QubesOS/qubes-issues#718

marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin