tests/qtest/virtio-net-failover: Use g_file_open_tmp() to create temporary file

tests/qtest/virtio-net-failover: Use g_file_open_tmp() to create temporary file

g_test_rand_int() must not be called before g_test_init(), otherwise
the glib will show a "g_rand_int: assertion 'rand != NULL' failed"
message in the log. So we could change the order here, but actually,
it's safer to use g_file_open_tmp() anyway, so let's use that function
now instead.

Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211222083652.776592-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>