tests/avocado/migration: Get find_free_port() from the ports

In upstream Avocado, the find_free_port() function is not available
from "network" anymore, but must be used via "ports", see:

 https://github.com/avocado-framework/avocado/commit/22fc98c6ff76cc55c48

To be able to update to a newer Avocado version later, let's use
the new way for accessing the find_free_port() function here.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220829121939.209329-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/avocado/migration.py

@@ -14,7 +14,7 @@
 from avocado_qemu import QemuSystemTest
 from avocado import skipUnless
 
-from avocado.utils import network
+from avocado.utils.network import ports
 from avocado.utils import wait
 from avocado.utils.path import find_command
 
@@ -57,7 +57,7 @@ def do_migrate(self, dest_uri, src_uri=None):
         self.assert_migration(source_vm, dest_vm)
 
     def _get_free_port(self):
-        port = network.find_free_port()
+        port = ports.find_free_port()
         if port is None:
             self.cancel('Failed to find a free port')
         return port