avocado.plugins.vm: Add option group for libvirt domain options

As it helps organizing the help output of the application.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #219 from lmr/vm-plugins-improvements-2nd-round

avocado.plugins.vm: Add option group for libvirt domain options
examples: Update hello example with recent changes.

Update the "Hello World!" plugin example to match
the recent changes that we've made in the plugin structure.
docs: Update documentation about loading external plugins.

Make it clear that we need to prefix every external plugin
with "avocado_" to load it. Also, using the new updated "Hello
World!" plugin example.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #221 from ruda/master

Update documentation regarding external plugins and the hello world plugin example.
multiplexer: Set root node name to empty string.

Get rid of "/root" for the root node and use empty string as name,
in order to avoid to prefix filters and to display information
with the /root prefix in every output.

The output will be just like this:

./scripts/avocado multiplex mux.yaml --filter-only /hw/cpu/arm --filter-out /os
Variants generated:
Variant 1:    /hw/cpu/arm, /hw/disk/scsi, /env/debug
Variant 2:    /hw/cpu/arm, /hw/disk/scsi, /env/prod
Variant 3:    /hw/cpu/arm, /hw/disk/virtio, /env/debug
Variant 4:    /hw/cpu/arm, /hw/disk/virtio, /env/prod

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