avocado.virt: Add defaults module

The defaults module will be used to store commonly
used default values in virt testing, such as:

* QEMU default paths
* Guest default paths

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt: Figure out guest image path if none provided

It is a common use case that people just want to use a particular
guest image file on a test. That can be done using the newly
introduced test runner option --guest-image-path. This value
is going to be passed to the test, and in case none is provided,
use what the virt.defaults module tells we should use.

With this we get extra flexibility and convenience running our
tests.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt: Add option --disable-restore-image-test

By default, restore image files if possible. As some
people might want to save time, give them that option
through command line flag --disable-restore-image-test.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt: Add option --disable-restore-image-job

Allow people to skip all uncompress guest image stages
if both --disable-restore-image-test and the new option,
--disable-restore-image-job are provided. If only the
former is provided, do the setup at plugin activation
stage. Print a 'Please wait...' message only if the
alternative output plugins (xunit and json) are not
claiming the application stdout.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.virt: Add virt related parser group

This will organize the virt options better, helping
the logical display of command line arguments.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt.qemu.devices: Simplify image path resolution code

Somehow, embarassingly I've missed this equivalent, much
more elegant construction. Thanks to Ruda and Cleber for
letting me realize my mistake.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.virt: Remove redundant 'dest' declarations

Argparse already creates those exact same destination
variables by default, rendering the explicit declarations
unnecessary.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #2 from lmr/sprint-13

Sprint 13