hw/ppc: Round up the decrementer interval when converting to ns

hw/ppc: Round up the decrementer interval when converting to ns

The rule of timers is typically that they should never expire before the
timeout, but some time afterward. Rounding timer intervals up when doing
conversion is the right thing to do.

Under most circumstances it is impossible observe the decrementer
interrupt before the dec register has triggered. However with icount
timing, problems can arise. For example setting DEC to 0 can schedule
the timer for now, causing it to fire before any more instructions
have been executed and DEC is still 0.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>