Engine: log total time to migration

Engine is logging the actual time that last migration took, but not the
total time that all the migration process took (ie with retries).

Bug-Url: https://bugzilla.redhat.com/1054143
Change-Id: Ib87bb88de09a61b9c3e3d4565deb23c9cb07bbf3
Signed-off-by: Shahar Havivi <shaharh@redhat.com>