utils_test.libvirt: Enhancements to MigrationTest class.

 * Record spent time of migration.
 * Allow to pass options for migration command.
 * Cleanup remote vm by gracefully=False to avoid
   unaccessible vm session.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.load_stress: Fix typo.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
utils_test.libvirt: Fix bugs of cross migration.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1585 from yumingfei/enhance_migration

utils_test.libvirt: Enhancements to MigrationTest class.