CostMonitoring: minor updates

Handle ROB status flags:

• remove SCHEDULED (prefetched) (DCM cached but not retrieved ROBs are not accessible to the HLT monitoring)
• rename DISABLED -> UNDEFINED (ROBs that are not on enabled list should not appear in monitoring)

Also, save robs status as unsigned short to avoid the type misinterpretation issues.

Tagging @mark @tamartin @wiedenma