vdsm: migration status structure refactoring

Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Change-Id: Iffff72af06c0d6d91968a6aa27780e16e77213ef
Reviewed-on: http://gerrit.ovirt.org/11913
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>