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

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>