net: virtual: Align virtual L2 with iface status upgrade

Align virtual L2 with interface state handling update. Introduce
net_virtual_enable() function, which gets called whenever a network
interface is brought up (operational). This, combined with already
existing net_virtual_disable() function, can be used to update the
carrier state on the virtual interface, based on the underlying
interface status.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>