tests/migration: fix memleak in wait_command/wait_command_fd

tests/migration: fix memleak in wait_command/wait_command_fd

Properly free each command resp to avoid memory leak.
ASAN shows memory leak stack:

Indirect leak of 2352520 byte(s) in 571 object(s) allocated from:
    #0 0x7f6ca3308d4e in __interceptor_calloc (/lib64/libasan.so.5+0x112d4e)
    #1 0x7f6ca3127a50 in g_malloc0 (/lib64/libglib-2.0.so.0+0x55a50)
    #2 0x557bf3c71d2b in qdict_new ../qobject/qdict.c:29
    #3 0x557bf3c9caba in parse_object ../qobject/json-parser.c:318
    #4 0x557bf3c9ce75 in json_parser_parse ../qobject/json-parser.c:580
    #5 0x557bf3c8c8cf in json_message_process_token ../qobject/json-streamer.c:92
    #6 0x557bf3c9ea59 in json_lexer_feed_char ../qobject/json-lexer.c:313
    #7 0x557bf3c9eeb5 in json_lexer_feed ../qobject/json-lexer.c:350
    #8 0x557bf3c4793a in qmp_fd_receive ../tests/qtest/libqtest.c:608
    #9 0x557bf3c47b58 in qtest_qmp_receive ../tests/qtest/libqtest.c:618
    #10 0x557bf3c44245 in wait_command ../tests/qtest/migration-helpers.c:59
    #11 0x557bf3c445cb in migrate_query_status ../tests/qtest/migration-helpers.c:108
    #12 0x557bf3c44642 in check_migration_status ../tests/qtest/migration-helpers.c:124
    #13 0x557bf3c447e7 in wait_for_migration_status ../tests/qtest/migration-helpers.c:148
    #14 0x557bf3c43b8f in test_migrate_auto_converge ../tests/qtest/migration-test.c:1243
    ......

Fix: 5e34005571af5

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Message-Id: <20201023061218.2080844-2-kuhn.chenqun@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>