qemu: Refactor qemuBuildNetworkDriveURI to take a virStorageSourcePtr
Instead of splitting out various fields, pass the complete structure and
let the function pick various things of it.
As one of the callers isn't using virStorageSourcePtr to store the data,
this patch adds glue code that fills the data into a dummy
virStorageSourcePtr before calling the func.
This change will help when adding new fields that need output processing
in the future.