restapi: fix NPE on vm statistics

The problem was that the getMigrationProgressPercent can return null, but this
case was not handled and the setting to long throw a NPE.

Fixed by explicitly handling the null.

Change-Id: Iee06e42c33a6ade371a84deb89d459586d82a9c3
Bug-Url: https://bugzilla.redhat.com/1103490
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>