Software should see timer counter wraparound only after IRQ being triggered.
This fixes regression introduced by the commit 5a50307b ("hw/ptimer: Perform
counter wrap around if timer already expired"), resulting in monotonic timer
jumping backwards on SPARC emulated machine running NetBSD guest OS, as
reported by Mark Cave-Ayland.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Message-id: 20160708132206.2080-1-digetx@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>