virtio/console: Implement reset

The virtio-console reset cancels all running jobs.

Unfortunately we don't have a good way to prevent the term polling thread
from getting in the way, read invalid data during reset and cause a
segfault. To prevent this, move all handling of the Rx queue in the
threadpool job.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>