[ffmemless] Add check for device existence to plugin loading

Ngfd fails to run in SDK as there is no ffmemless device, and
a sink failing init will stop whole ngfd from working. Added
a check to plugin load for the device existance, so we don't
even load the plugin if we can't run it.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>