migration: add vsock as data channel support

The vsock channel is more widely use in some new features, for example,
the Nitro/Enclave. It can also be used as the migration channel.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Message-Id: <20200806074030.174-3-longpeng2@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>