gtk: overwrite the console.c char driver

Instead of registering a vc handler to allocate the Gtk VC Chardev,
overwrite the console.c char driver.

A later patch, when switching to QOM, will register a default console vc
QOM class if none has been registered before.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>