virtio-console: Check if chardev backends available before calling into them

For the callback functions invoked by the virtio-serial-bus code, check
if we have chardev backends registered before we call into the chardev
functions.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
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>
