tests/avocado: start PhoneServer upfront

tests/avocado: start PhoneServer upfront

Race conditions can happen with the current code, because the port that
was available might not be anymore by the time the server is started.

By setting the port to 0, PhoneServer it will use the OS default
behavior to get a free port, then we save this information so we can
later configure the guest.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Beraldo Leal <bleal@redhat.com>
Message-Id: <20220311171127.2189534-1-bleal@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>