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>