migration: fix cleanup_bh leak on resume

Since commit 8c6b0356 ("util/async:
make bh_aio_poll() O(1)"), migration-test reveals a leak:

QTEST_QEMU_BINARY=x86_64-softmmu/qemu-system-x86_64
tests/qtest/migration-test  -p /x86_64/migration/postcopy/recovery
tests/qtest/libqtest.c:140: kill_qemu() tried to terminate QEMU
process but encountered exit status 1 (expected 0)

=================================================================
==2082571==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x7f25971dfc58 in __interceptor_malloc (/lib64/libasan.so.5+0x10dc58)
    #1 0x7f2596d08358 in g_malloc (/lib64/libglib-2.0.so.0+0x57358)
    #2 0x560970d006f8 in qemu_bh_new /home/elmarco/src/qemu/util/main-loop.c:532
    #3 0x5609704afa02 in migrate_fd_connect
/home/elmarco/src/qemu/migration/migration.c:3407
    #4 0x5609704b6b6f in migration_channel_connect
/home/elmarco/src/qemu/migration/channel.c:92
    #5 0x5609704b2bfb in socket_outgoing_migration
/home/elmarco/src/qemu/migration/socket.c:108
    #6 0x560970b9bd6c in qio_task_complete /home/elmarco/src/qemu/io/task.c:196
    #7 0x560970b9aa97 in qio_task_thread_result
/home/elmarco/src/qemu/io/task.c:111
    #8 0x7f2596cfee3a  (/lib64/libglib-2.0.so.0+0x4de3a)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200325184723.2029630-2-marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>