migration: Trace improvements

migration: Trace improvements

A couple of improvements to tracing that have come out of helping
people with migration problems:
  * vmstate_n_elems trace the count/name - for when you have problems
    getting array counts right
  * vmstate_subsection_load_bad - add the idstr, for when you receive a
    subsection you weren't expecting.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1465896986-16132-1-git-send-email-dgilbert@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>