tests/avocado: Cancel BootLinux tests in case there is no free port

The BootLinux tests are currently failing with an ugly python
stack trace on my RHEL8 system since they cannot get a free port
(likely due to the firewall settings on my system). Let's properly
check the return value of find_free_port() instead and cancel the
test gracefully if it cannot get a free port.

Message-Id: <20220228114325.818294-1-thuth@redhat.com>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/avocado/avocado_qemu/__init__.py

@@ -603,6 +603,8 @@ def prepare_cloudinit(self, ssh_pubkey=None):
         try:
             cloudinit_iso = os.path.join(self.workdir, 'cloudinit.iso')
             self.phone_home_port = network.find_free_port()
+            if not self.phone_home_port:
+                self.cancel('Failed to get a free port')
             pubkey_content = None
             if ssh_pubkey:
                 with open(ssh_pubkey) as pubkey: