jqueue: Ensure only accepted priorities are allowed for submitting jobs

Quoting the design document: “Submitted opcodes can have one of the priorities
listed below. Other priorities are reserved for internal use”. Submitting jobs
at priority -20 should not be allowed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>