avocado.process: Make processes to fail when interrupted by user

Some tests, such as the trinity test, may pass even
when interrupted (Ctrl+C), and that is counter intuitive
with the expected semantic of Ctrl+C. So, even if a command
passed, let's raise a CmdError exception if interrupted.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.vm: Pass '-' to json plugin

So that the avocado command does produce its
intended output. Fixes a small bug introduced with
the simplification of the json and xunit plugins.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #162 from lmr/force-fail-ctrl-c

avocado.process: Make processes to fail when interrupted by user