When the tests are executed and rsync is not installed on the machine executing
the tests, then vagrant-libvirt will automatically fallback to virtiofs or
9p. Both of these perform the following call to check the libvirt version:
libvirt_version = machine.provider.driver.connection.client.libversion
However, this mock was not setup and thus those tests would fail.
This fixes https://github.com/vagrant-libvirt/vagrant-libvirt/issues/1415