libxc/progress: Extend the progress interface

libxc/progress: Extend the progress interface

Progress information is logged via a different logger to regular libxc log
messages, and currently can only express a range.  However, not everything
which needs reporting as progress comes with a range.  Extend the interface to
allow reporting of a single statement.

The programming interface now looks like:
  xc_set_progress_prefix()
    set the prefix string to be used
  xc_report_progress_single()
    report a single action
  xc_report_progress_step()
    report $X of $Y

The new programming interface is implemented in a compatible way with the
existing caller interface (by reporting a single action as "0 of 0").

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>