virFDStream: Add option for delete file after it's opening

virFDStream: Add option for delete file after it's opening

This is needed if we want to transfer a temporary file. If the
transfer is done with iohelper, we might run into a race condition,
where we unlink() file before iohelper is executed.

* src/fdstream.c, src/fdstream.h,
  src/util/iohelper.c: Add new option
* src/lxc/lxc_driver.c, src/qemu/qemu_driver.c,
  src/storage/storage_driver.c, src/uml/uml_driver.c,
  src/xen/xen_driver.c: Expand existing function calls