engine: migration progress

The goal is to show in webadmin the migration progress of the VM.

VDSM already returns as a statistic the migration progress.

This is the engine part of the migration progress feature.

Change-Id: I5e909b1577a26f83bf46ff14694f0ca66c32076a
Bug-Url: https://bugzilla.redhat.com/1083049
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>