virttest.libvirt_vm: Add domjobabort method for VM class.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt.tests: Add testcases for migration of multiple vms.

Migrate multiple vms with these features:
1.Orderly or simultaneous migration.
2.Simple live migration or combination options.
3.Different migration method.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>