avocado.virt.qemu.machine: Encode videos upon VM object termination

As soon as the machine is powered off, let's create a
video from the screenshots generated by the screendump
thread. In order to not encode videos unnecessarily
during migration, pass a migrate param to encode videos
and power off/screendump thread end methods to skip
video encoding in such ocasions.

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>
Bugfix: avocado.virt.qemu.machine: Close serial console tail process

Close the serial console tail process, because that will
prevent processes from lying around after an avocado virt
test is done.

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