tests.migrate_stress: Set autologin for remote host.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
tests.migrate_stress: Do not check IP in cross migration.

In cross migration, vm in list may be migrated back after pairs of
migration and it's not easy to track them, so skipping IP checking here.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>