libvirt.tests: Fix parameter error of migrate-setmaxdowntime.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Fix migrate stress problems.

 * Vms will be popped in cross migration, so creating them again
   for cleanup.
 * If local vms use lan, geting address would be failed on remote
   host, so storing ips before migration.
 * Fix stress_args parameter.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
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>
Merge pull request #146 from yumingfei/migration_enhancements

Libvirt.tests: Migration tests' enhancements