tests/migration: parse the thread-id key of CpuInfoFast

thread_id in CpuInfoFast is deprecated, parse thread-id instead
after execute qmp query-cpus-fast. fix this so that test can
go smoothly.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Message-Id: <584578c0a0dd781cee45f72ddf517f6e6a41c504.1622729934.git.huangy81@chinatelecom.cn>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>