avocado.plugins: Fix bug 'Namespace' object has no attribute

Fix problem when running Avocado with subcommand other than run:

$ avocado plugins
Could not activate plugin 'run_vm': 'Namespace' object has no attribute 'vm'
Could not activate plugin 'json': 'Namespace' object has no attribute 'json_output'
Could not activate plugin 'xunit': 'Namespace' object has no attribute 'xunit_output'
Could not activate plugin 'journal': 'Namespace' object has no attribute 'journal'
Plugins loaded:
    datadir      - Implements the avocado 'datadir' subcommand (Enabled)
    json         - JSON output (Enabled)
    journal      - Test journal (Enabled)
    multiplexer  - Implements the avocado 'multiplex' subcommand. (Enabled)
    plugins_list - Implements the avocado 'plugins' subcommand (Enabled)
    run_vm       - Run tests on a Virtual Machine (Enabled)
    sysinfo      - Collect system information (Enabled)
    test_lister  - Implements the avocado 'list' subcommand (Enabled)
    test_runner  - Implements the avocado 'run' subcommand (Enabled)
    xunit        - xUnit output. (Enabled)

Signed-off-by: Ruda Moura <rmoura@redhat.com>