engine: Fix coverity complaint

CID 1223220 (#1 of 1): Dereference null return value (NULL_RETURNS)
null_method_call: Calling a method on null object jobType.

In addition, the job type is not allowed to be null in vm_job table
hence a default value should be set for it.

Change-Id: If67422d839f16be850b91d9bdfdf58f7359d4672
Signed-off-by: Moti Asayag <masayag@redhat.com>