virt/steal_governor: Add control knobs for handling steal values

These are the knobs to control the steal_governor.

interval_ms:
How often steal governor checks for steal time.
(Default: 1000 i.e 1 second)
This controls how fast steal governor driver reacts to changes to
the contention of physical CPUs.
Can be set between 100 to 100000. i.e. 100ms to 100seconds.
100ms is kept as minimum to ensure few meaningful steal values
accumulate even with HZ=100.

low_threshold:
lower threshold value in percentage * 100.
(Default: 200, i.e 2% steal is considered as low threshold)
This determines what values should be considered as nil/no steal values.
When steal governor see steal ratio is below or equal to this value, it
will increase the preferred CPUs by 1 core. Having value as zero
might cause oscillations

high_threshold:
higher threshold value in percentage * 100
(Default: 500, i.e 5% steal is considered as high threshold)
This determines what values should be considered as high steal values.
When steal governor sees steal ratio is higher than this value, it will
reduce the preferred CPUs by 1 core.

module_param_cb methods are used to do the validation checks.
This helps to ensure one configures sane values.
Since low and high are dependent, that check is done at module init.

Notes:
- Parameters values can't be changed at runtime. One has to unload
  the module and change it. Hence recommended to build it as module.
- Default values may not work well for all configurations. Tune it
  according to the system under test.

Documentation is available at: Documentation/driver-api/steal-governor.rst

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260928053728.797539-12-sshegde@linux.ibm.com

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