vm: split migration completion in smaller methods

Refactor _waitForIncomingMigrationFinish in smaller
methods for clarity and to prepare for better
timeout handling. The method was renamed for better
clarity: in the case of dehibernation it wasn't
waiting for anything.
No functional changes.

Change-Id: Ic469814afc607951870f4171499cef958855fc54
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28509
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>