kvm tools: virtio-mmio: init_ioeventfd should use MMIO for ioeventfd__add_event()

This patch fixes a bug that vtirtio_mmio_init_ioeventfd() passed a wrong
value when it invoked ioeventfd__add_event(). True value of 2nd parameter
indicates the eventfd uses PIO bus which is used by virito-pci, however,
for virtio-mmio, the value should be false.

Signed-off-by: Ying-Shiuan Pan <yspan@itri.org.tw>
Signed-off-by: Pekka Enberg <penberg@kernel.org>