PPC: Make BookE FIT/WDT timers more lazy

Today we fire FIT and WDT timer events every time the respective bit
position in TB flips from 0 -> 1.

However, there is no need to do this if the end result would be that
we're changing a TSR bit that is set to 1 to 1 again. No guest visible
change would have occured.

So whenever we see that the TSR bit to our timer is already set, don't
even bother to update the timer that would potentially fire it off.

However, we do need to make sure that we update our timer that notifies
us of the TB flip when the respective TSR bit gets unset. In that case
we do care about the flip and need to notify the guest again. So add
a callback into our timer handlers when TSR bits get unset.

This improves performance for me when the guest is busy processing things.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - add parenthesis
  - use QEMUTimer *
PPC: BookE: Make FIT/WDT timers at best millisecond grained

The default granularity for the FIT timer on 440 is on every 0x1000th
transition of TB from 0 to 1. Translated that means 48828 times a second.

Since interrupts are quite expensive for 440 and we don't really care
about the accuracy of the FIT to that significance, let's force FIT and
WDT to at best millisecond granularity.

This basically restores behavior as it was in QEMU 1.6, where timers
could only deal with millisecond granularities at all.

This patch greatly improves performance with the 440 target and restores
roughly the same performance level that QEMU 1.6 had for me.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - s/milliseconds/milliseconds/g