libxl: event handling: Add an assert to egc_run_callbacks

libxl: event handling: Add an assert to egc_run_callbacks

Check that the ao is still live when we are about to running some of
its callbacks.

This reveals an existing bug in libxl which is exercised by libvirt,
converting
   libvirtd: libxl_event.c:1792: libxl__ao_complete_check_progress_reports: Assertion `ao->in_initiator' failed.
into
   libvirtd: libxl_event.c:1338: egc_run_callbacks: Assertion `aop->ao->magic == 0xA0FACE00ul' failed.

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>