libvirt.tests: Enhancements of stress migration tests.

* Support migration in thread for expanding.
* Fix bugs and some pylint problems.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Add cross migration test for virsh_migrate_stress.

Cross migration:
local     =====      remote
           <-         vm1
vm2        ->
~~
           <-         vm2
vm3        ->

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Add migration testcases when vms are booting.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Expand stress type for migration stress test.

 * Seperate cpu and memory stress of stress tool.
 * Add timeout parameter for migration threads.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Running stress tool on host for stress migration.

Do migration when stress tool is running on host:
1.Half of total memory is consumed.
2.Host is short of memory(Free memory is not enough for vms.)

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Add network check after migration for stress migration test.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.testcases: Enhancements of stress migration.

* Use common parameters for remote info and vm info.
* Use specific parameters for stress tool.
* Fix pylint complaints for stress migration module.
* Fix pep8 errors.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #840 from yumingfei/stress_migration

libvirt.tests: Add migration test for virsh_migrate_stress.