Enhance the streams helper to support plain file I/O

Enhance the streams helper to support plain file I/O

The O_NONBLOCK flag doesn't work as desired on plain files
or block devices. Introduce an I/O helper program that does
the blocking I/O operations, communicating over a pipe that
can support O_NONBLOCK

* src/fdstream.c, src/fdstream.h: Add non-blocking I/O
  on plain files/block devices
* src/Makefile.am, src/util/iohelper.c: I/O helper program
* src/qemu/qemu_driver.c, src/lxc/lxc_driver.c,
  src/uml/uml_driver.c, src/xen/xen_driver.c: Update for
  streams API change