replay: improve interrupt handling

This patch improves interrupt handling in record/replay mode.
Now "interrupt" event is saved only when cc->cpu_exec_interrupt returns true.
This patch also adds missing return to cpu_exec_interrupt function.

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20170124071708.4572.64023.stgit@PASHA-ISP>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
