engine: Add new event notifiers

Add notification support for the following existing event types:
 - AuditLogType.VM_DOWN_ERROR
 - AuditLogType.VDS_INITIATED_RUN_VM_FAILED

Since both of these events relate specifically to problems running a VM,
place them in the EventNotificationEntity.Vm category.

Signed-off-by: Adam Litke <alitke@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1009005
Change-Id: Ib1ad9508893adb11b745b86747bf368115eaaea2