PPC/KVM: make iothread work

When running with --enable-io-thread the timer we have doesn't help,
because it doesn't wake up the CPU thread. So instead we need to
actually kick it.

While at it I refined the logic a bit to not dumbly trigger a timer
every 500ms, but rather do it more often after an interrupt got injected.
If there's no level based interrupt to be expected, we don't need the
timer anyways.

This makes qemu-system-ppc with --enable-io-thread work when using KVM.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>