backend: Add richer formatting of migration duration

Upon migration completion the events showed migration duration only in
seconds (e.g. 1244 sec).

Now we display it in the format "1 hour, 15 minutes, 59 seconds".

Change-Id: Ic6e8901be66480c7212fa3a98a28a332d13f13e8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1092884
Signed-off-by: Martin Betak <mbetak@redhat.com>