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This field represents mutually exclusive states, namely

- IRQENTRY_INBAND_UNSTALLED
- IRQENTRY_INBAND_STALLED
- IRQENTRY_OOB

Encodes them as enum and test against them, rather than against state
bits that suggest they could be combined.

Also flip the inverted naming of INBAND_STALLED vs. INBAND_UNSTALLED:
Only when we entered under INBAND_UNSTALLED, certain actions need to be
taken on exit.

Finally, document the stage_info field of irqentry_state.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>