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>