tests/qtest/migration-test: Don't leak QTestState in test_multifd_tcp_cancel()

In test_multifd_tcp_cancel() we create three QEMU processes: 'from',
'to' and 'to2'.  We clean up (via qtest_quit()) 'from' and 'to2' when
we call test_migrate_end(), but never clean up 'to', which results in
this leak:

Direct leak of 336 byte(s) in 1 object(s) allocated from:
    #0 0x55e984fcd328 in __interceptor_calloc (/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/asan/tests/qtest/migration-test+0x22f328) (BuildId: 710d409b68bb04427009e9ca6e1b63ff8af785d3)
    #1 0x7f0878b39c50 in g_malloc0 debian/build/deb/../../../glib/gmem.c:161:13
    #2 0x55e98503a172 in qtest_spawn_qemu tests/qtest/libqtest.c:397:21
    #3 0x55e98502bc4a in qtest_init_internal tests/qtest/libqtest.c:471:9
    #4 0x55e98502c5b7 in qtest_init_with_env tests/qtest/libqtest.c:533:21
    qemu#5 0x55e9850eef0f in test_migrate_start tests/qtest/migration-test.c:857:11
    qemu#6 0x55e9850eb01d in test_multifd_tcp_cancel tests/qtest/migration-test.c:3297:9
    qemu#7 0x55e985103407 in migration_test_wrapper tests/qtest/migration-helpers.c:456:5

Call qtest_quit() on 'to' to clean it up once it has exited.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Fabiano Rosas <[email protected]>
Signed-off-by: Fabiano Rosas <[email protected]>

Author: pm215

tests/qtest/migration-test.c

@@ -3242,6 +3242,7 @@ static void test_multifd_tcp_cancel(void)
     /* Make sure QEMU process "to" exited */
     qtest_set_expected_status(to, EXIT_FAILURE);
     qtest_wait_qemu(to);
+    qtest_quit(to);
 
     args = (MigrateStart){
         .only_target = true,