[virtio][block] simplify guest signaling
[virtio][block] simplify guest signaling

Trigger guest irqs from a single place.

Signed-off-by: Serban Iorga <4cd75ae1c2f5ea126376f5daa1ce9f5f3016cb24@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker