migration: fix downtime limit auto-increase

migration: fix downtime limit auto-increase

485449e37 ("qmp: use migrate-set-parameters in favor of deprecated options")
changed the initial "migrate_set_downtime" QMP call to the more recent
"migrate-set-parameters", but forgot to do so for the auto-increase code
further below.

Since the units of the two calls don't match, this would have caused the
auto-increase to increase the limit to absurd levels as soon as it kicked
in (ms treated as s).

Update the second call to the new version as well, and while at it remove
the unnecessary "defined()" check for $migrate_downtime, which is always
initialized from the defaults anyway.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>