vhost: potential crash in map_guest_mem using debug image

vhost: potential crash in map_guest_mem using debug image

map_guest_mem may be called from worker-thread/dataplane. It has a call
to vlib_log and may crash inside vlib_log's ASSERT statement

  /* make sure we are running on the main thread to avoid use in dataplane
     code, for dataplane logging consider use of event-logger */
  ASSERT (vlib_get_thread_index () == 0);

The fix is to convert the vlib_log call in map_guest_map to event logger

Change-Id: Iaaf6d86782aa8a18d25e0209f22dc31f04668d56
Signed-off-by: Steven Luong <sluong@cisco.com>
(cherry picked from commit b0789106cb4f4c7ac529c02a5ff1ac346f0913dd)