Function build_vtl2_storage_backing_physical_devices

pub fn build_vtl2_storage_backing_physical_devices(
    devices: Vec<Vtl2StorageBackingDeviceBuilder>,
) -> Option<PhysicalDevices>

VTL2 Settings wraps the list of backing devices into a PhysicalDevices type, with subtlely different fields depending on whether there is one or multiple devices. This helper builds that wrapper type.