tests/migration: fix "downtime_limit" type when "migrate-set-parameters"

migrate-set-parameters parse "downtime_limit" as integer type when
execute "migrate-set-parameters" before migration, and, the unit
dowtime_limit is milliseconds, fix this two so that test can go
smoothly.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Message-Id: <31d82df24cc0c468dbe4d2d86730158ebf248071.1622729934.git.huangy81@chinatelecom.cn>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>