Fixes#140
`virtcontainers` had been moved from `github.com/containers/virtcontainers` to
`github.com/kata-containers/runtime/virtcontainers`, so we should remove legacy dependencies
of `github.com/containers/virtcontainers` from go imports and vendors.
Also some documents need to be modified.
Note: vendor is updated with `dep` tool with following command:
`$ dep ensure -update github.com/containers/virtcontainers`
Signed-off-by: Zhang Wei <zhangwei555@huawei.com>