model: Correct timing of RT_LAST_KNOWN_DELTA_CYCLE

model: Correct timing of RT_LAST_KNOWN_DELTA_CYCLE

According to the spec, vhpiCbLastKnownDeltaCycle callbacks can create
further delta cycles:

> h) If the next simulation cycle will be a delta cycle, the remainder of
> step h) is skipped. Otherwise, the following actions occur in the
> indicated order:
>
>   1. Each registered and enabled `vhpiCbLastKnownDeltaCycle` and
>      `vhpiCbRepLastKnownDeltaCycle` callback is executed. T_n is
>      recalculated according to the rules of 14.7.5.1.
>   2. If the next simulation cycle will be a delta cycle, the remainder
>      of step h) is skipped.
>   3. ...

Fire RT_LAST_KNOWN_DELTA_CYCLE before disabling delta cycles to allow
this to happen.