kvm tools: ioeventfd: replace bool parameters to __add_event with flags

A recent fix to virtio MMIO (72a7541ce305 ["kvm tools: virtio-mmio:
init_ioeventfd should use MMIO for ioeventfd__add_event()"]) highlighted
the confusing parameters expected by ioeventfd__add_event.

As per Pekka's suggestion, replace the bool parameters to this function
with a single `flags' argument instead.

Cc: Ying-Shiuan Pan <yingshiuan.pan@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>