avocado-virt.spec: Add avocado-virt-video subpackage

Turn the video library into a subpackage, that will
let users to install the video functionality if they
want so.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt.qemu.machine: Change video and screendump defaults

Turn both features off by default, since we want to keep avocado
simple and fast.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado-virt: Add --take-screendumps and --record-videos options to plugin

Make both the screendump feature and the record video feature
optional, and turn them into avocado command line options.
This way people have flexibility to use the features, while
keeping avocado simple and fast.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>