remove vtape autodetection in init_dev

Annoyingly, the vtape code was automatically
activated in init_dev without any notice if
device->device_name was a regular file.

We now deactivated that so that you explicitly
have to set

   "DeviceType = Vtape"

if you wish to use vtape instead of the stupid
autodetection.

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>
Make clear when vtape is compiled in.
Show an error when an unknown device type is configured.