x86/cpuidle: switch to uniform meaning of "max_cstate="

x86/cpuidle: switch to uniform meaning of "max_cstate="

While the MWAIT idle driver already takes it to mean an actual C state,
the ACPI idle driver so far used it as a list index. The list index,
however, is an implementation detail of Xen and affected by firmware
settings (i.e. not necessarily uniform for a particular system).

While touching this code also avoid invoking menu_get_trace_data()
when tracing is not active. For consistency do this also for the
MWAIT driver.

Note that I'm intentionally not adding any sorting logic to set_cx():
Before and after this patch we assume entries to arrive in order, so
this would be an orthogonal change.

Take the opportunity and add minimal documentation for the command line
option.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>