tests/migration: Reduce autoconverge initial bandwidth

When using max-bandwidth=~100Mb/s, this test fails on Travis-CI
s390x when configured with --disable-tcg:

  $ make check-qtest
    TEST    check-qtest-s390x: tests/qtest/boot-serial-test
  qemu-system-s390x: -accel tcg: invalid accelerator tcg
  qemu-system-s390x: falling back to KVM
    TEST    check-qtest-s390x: tests/qtest/pxe-test
    TEST    check-qtest-s390x: tests/qtest/test-netfilter
    TEST    check-qtest-s390x: tests/qtest/test-filter-mirror
    TEST    check-qtest-s390x: tests/qtest/test-filter-redirector
    TEST    check-qtest-s390x: tests/qtest/drive_del-test
    TEST    check-qtest-s390x: tests/qtest/device-plug-test
    TEST    check-qtest-s390x: tests/qtest/virtio-ccw-test
    TEST    check-qtest-s390x: tests/qtest/cpu-plug-test
    TEST    check-qtest-s390x: tests/qtest/migration-test
  **
  ERROR:tests/qtest/migration-test.c:1229:test_migrate_auto_converge: 'got_stop' should be FALSE
  ERROR - Bail out! ERROR:tests/qtest/migration-test.c:1229:test_migrate_auto_converge: 'got_stop' should be FALSE
  make: *** [tests/Makefile.include:633: check-qtest-s390x] Error 1

Per David Gilbert, "it could just be the writing is slow on s390
and the migration thread fast; in which case the autocomplete
wouldn't be needed. Perhaps we just need to reduce the bandwidth
limit."

Tuning the threshold by reducing the initial bandwidth makes the
autoconverge test pass.

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200323184015.11565-1-philmd@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>