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>

tests/migration/guestperf/engine.py

@@ -153,7 +153,7 @@ def _migrate(self, hardware, scenario, src, dst, connect_uri):
                            max_bandwidth=scenario._bandwidth * 1024 * 1024)
 
         resp = src.command("migrate-set-parameters",
-                           downtime_limit=scenario._downtime / 1024.0)
+                           downtime_limit=scenario._downtime)
 
         if scenario._compression_mt:
             resp = src.command("migrate-set-capabilities",