save: let iohelper handle inherited fd

save: let iohelper handle inherited fd

Rather than making the iohelper subject to a race in reopening
the file, it is nicer to pass an already-open fd by inheritance.

The old synopsis form must continue to work - if someone updates
their libvirt package and installs a new libvirt_iohelper but
without restarting the old libvirtd daemon, then the daemon can
still make calls using the old syntax but the new iohelper.

* src/util/iohelper.c (runIO): Split code for open...
(prepare): ...to new function.
(usage): Update synopsis.
(main): Allow alternate calling form.
* src/fdstream.c (virFDStreamOpenFileInternal): Use alternate form.