Rename the `vsockport` in the pre-mig command for virtio-serial

[gaojiaqi7]

[jyao1]

@wei-w-wang, when linux team can resolve this?

[wei-w-wang]

Maybe Q2'14 (when the related QEMU patches can be sent out). It's not urgent to change. Current BKC supports both virtio-vsock and virtio-seiral based MigTD. Keeping it allows us to reuse one script for both cases.