engine: Enable per-VM configuration of migration downtime

Added migration_downtime field to vm_static table and
Integer migrationDowntime property to VmBase representing
maximum number of milliseconds for downtime during live migration.

In case this value is null the new ConfigValue
DefaultMaximumMigrationDowntime value is used.

In case either the vm field or the aforementioned default value is "0"
(integer zero) no value will be sent to VDSM and effectively the default
VDSM behavior will be enacted.

Change-Id: Ia16f66540497d20178efae6989710000110a81d6
Signed-off-by: Martin Betak <mbetak@redhat.com>