qemu: command: Report error when formatting network source with protocol _NONE

The function that formats the string for network drives would return
error code but did not set the error message when called on storage
source with VIR_STORAGE_NET_PROTOCOL_LAST or _NONE.

Report an error in this case if it would ever be called in that way.

src/qemu/qemu_command.c

@@ -3264,6 +3264,9 @@ qemuBuildNetworkDriveURI(virStorageSourcePtr src,
 
         case VIR_STORAGE_NET_PROTOCOL_LAST:
         case VIR_STORAGE_NET_PROTOCOL_NONE:
+            virReportError(VIR_ERR_INTERNAL_ERROR,
+                           _("Unexpected network protocol '%s'"),
+                           virStorageNetProtocolTypeToString(src->protocol));
             goto cleanup;
     }