do not use qemu_event_increment outside qemu_notify_event

qemu_notify_event in the non-iothread case is only stopping the current
CPU.  However, if the CPU is idle and the main loop is in the select
call then a call to qemu_event_increment is needed too (as done in
host_alarm_handler).  Since in general one doesn't know whether the CPU
is executing or not, it is a safe bet to always do qemu_event_increment.

Another way to see it: after this patch qemu_event_increment is the
"common part" of qemu_notify_event for both the CONFIG_IOTHREAD and
!CONFIG_IOTHREAD cases, which makes sense.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
