uterm: vt: register input-cb for all VTs

uterm: vt: register input-cb for all VTs

This modifies the generic VT layer to register input-cbs for all VT types
and dispatch the event to the correct handler.
This will allow us to handle VT-switches for real VTs ourself instead of
relying on VT input.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>