kvmtool: Fix length of ioevent for VIRTIO_PCI_QUEUE_NOTIFY

VIRTIO_PCI_QUEUE_NOTIFY is 16-bit and iowrite16 is used in
drivers/virtio/virtio_pci.c to notify the other side.

If the size doesn't match notification via mmio write will fail.

Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>