libvirt.tests: Add testcases for migrating with virtio-scsi devices.

1.Migrating with attached scsi disks on vm(file type and disk type)
2.Attaching scsi disks when migrating(file type and disk type)

Rely on https://github.com/autotest/virt-test/pull/1811

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>