zvsh: make Shell accept custom arg namespace
zvsh: make Shell accept custom arg namespace

This adds an optional arg to the Shell constructor so that the
`argparse.Namespace` object can be passed in. This is useful for
utilizing alternate arg parsers.

zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zerovm-cli