tests/functional/test_virtio_balloon: Only use KVM for running this test

tests/functional/test_virtio_balloon: Only use KVM for running this test

The virtio_balloon test is currently hanging for unknown reasons
when being run on the shared gitlab CI runners (which don't provide
KVM, thus it's running in TCG mode there). All other functional tests
that use the same asset (the Fedora 31 kernel) have already been
marked to work only with KVM in the past, so those other tests are
skipped on the shared gitlab CI runners. As long as the problem isn't
fully understood and fixed, let's do the same with the virtio_balloon
test to avoid that the CI is failing here.

Message-ID: <20250307063904.1081961-1-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>