GDB: deal with fatal signals and generate core dumps

This adds two related features:

1) dealing with fatal signals such as SIGSEGV
2) generating core dumps automatically (disabled by default) if requested

So now the behaviour on fatal signals (SISEGV and SIGABRT) for now cause
the test to be skipped. Other ways to deal with this proved to be complex
and not really effective.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #218 from clebergnu/gdb_fatal_signals_v2

GDB: deal with fatal signals and generate core dumps (v2)
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>