vm: report migration progress in VmStats

add migration progress information in the
VM Statistics.

This information is passed to engine, which can now
in turn inform the user about the migration status.

Change-Id: I35e40663921b2d73a9b49727c61b8b1f4dacdb9a
Bug-Url: https://bugzilla.redhat.com/967466
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24465
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>