kvm: Mock .slapos-resource for tap_ipv4_addr

Since
nexedi/slapos.core@a8917a3c
and slapos.core https://lab.nexedi.com/nexedi/slapos.core/tags/1.7.4 the
tap_ipv4_addr value is empty by default on the servers, especially test nodes.

To not rely on what is provided on the test environment simply mock the
configuration of the machine so that the test is able to prove profile
functionality.