net: virtual: Pass status to caller when starting/stopping interface

When virtual interface is being started or stopped so when the
interface is brought up or down, check the return code and pass
it to net_if_up() or net_if_down() calls.
This helps to avoid printing success status even if the interface
up/down failed at the virtual interface level.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>