libxl: event handling: Break out ao_work_outstanding

libxl: event handling: Break out ao_work_outstanding

Break out the test in libxl__ao_complete_check_progress_reports, into
ao_work_outstanding, which reports false if either (i) the ao is still
ongoing or (ii) there is a progress report (perhaps on a different
thread's callback queue) which has yet to be reported to the
application.

No functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Jim Fehlig <jfehlig@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>