migration: fix QEMUFile leak

Spotted by ASAN while running:

$ tests/migration-test -p /x86_64/migration/postcopy/recovery

=================================================================
==18034==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 33864 byte(s) in 1 object(s) allocated from:
    #0 0x7f3da7f31e50 in calloc (/lib64/libasan.so.5+0xeee50)
    #1 0x7f3da644441d in g_malloc0 (/lib64/libglib-2.0.so.0+0x5241d)
    #2 0x55af9db15440 in qemu_fopen_channel_input /home/elmarco/src/qemu/migration/qemu-file-channel.c:183
    #3 0x55af9db15413 in channel_get_output_return_path /home/elmarco/src/qemu/migration/qemu-file-channel.c:159
    #4 0x55af9db0d4ac in qemu_file_get_return_path /home/elmarco/src/qemu/migration/qemu-file.c:78
    #5 0x55af9dad5e4f in open_return_path_on_source /home/elmarco/src/qemu/migration/migration.c:2295
    #6 0x55af9dadb3bf in migrate_fd_connect /home/elmarco/src/qemu/migration/migration.c:3111
    #7 0x55af9dae1bf3 in migration_channel_connect /home/elmarco/src/qemu/migration/channel.c:91
    #8 0x55af9daddeca in socket_outgoing_migration /home/elmarco/src/qemu/migration/socket.c:108
    #9 0x55af9e13d3db in qio_task_complete /home/elmarco/src/qemu/io/task.c:158
    #10 0x55af9e13ca03 in qio_task_thread_result /home/elmarco/src/qemu/io/task.c:89
    #11 0x7f3da643b1ca in g_idle_dispatch gmain.c:5535

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180925092245.29565-1-marcandre.lureau@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2280,7 +2280,10 @@ static void *source_return_path_thread(void *opaque)
              */
             if (postcopy_pause_return_path_thread(ms)) {
                 /* Reload rp, reset the rest */
-                rp = ms->rp_state.from_dst_file;
+                if (rp != ms->rp_state.from_dst_file) {
+                    qemu_fclose(rp);
+                    rp = ms->rp_state.from_dst_file;
+                }
                 ms->rp_state.error = false;
                 goto retry;
             }