virtio-console: Properly initialise class methods

The earlier code really was a hack: initialising class methods in an
object init function as noted by Anthony.

The motivation for that was to not have the virtio-serial-bus call into
the callback functions if there was no chardev backend registered.
However, that really wasn't a worthwhile optimisation, and definitely
not one that was well-implemented.  Get rid of it.

Reported-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
