Hirsute - udevadm

# attach
KERNEL[319.020043] add      /devices/css0/0.0.0005 (css)
KERNEL[319.020088] add      /devices/css0/0.0.0005/0.0.0005 (ccw)
KERNEL[319.020103] bind     /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [319.022297] add      /devices/css0/0.0.0005 (css)
UDEV  [319.022802] add      /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [319.023039] bind     /devices/css0/0.0.0005/0.0.0005 (ccw)
KERNEL[319.025073] add      /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
UDEV  [319.025527] add      /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
KERNEL[319.027524] add      /devices/virtual/bdi/252:32 (bdi)
UDEV  [319.028389] add      /devices/virtual/bdi/252:32 (bdi)
KERNEL[319.048685] add      /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
KERNEL[319.048743] bind     /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
UDEV  [319.072936] add      /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
UDEV  [319.075862] bind     /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)

# detach
<no entry>

Groovy - udevadm

# attach
KERNEL[719.986637] add      /devices/css0/0.0.0005 (css)
KERNEL[719.986669] add      /devices/css0/0.0.0005/0.0.0005 (ccw)
KERNEL[719.986685] bind     /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [719.988667] add      /devices/css0/0.0.0005 (css)
KERNEL[719.992750] add      /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
KERNEL[719.992757] add      /devices/virtual/bdi/252:32 (bdi)
UDEV  [719.993298] add      /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [719.993520] bind     /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [719.994097] add      /devices/virtual/bdi/252:32 (bdi)
UDEV  [719.995568] add      /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
KERNEL[720.009523] add      /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
KERNEL[720.009544] bind     /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
UDEV  [720.058301] add      /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
UDEV  [720.059128] bind     /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)

# detach
KERNEL[780.673663] remove   /devices/virtual/bdi/252:32 (bdi)
KERNEL[780.673928] remove   /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
UDEV  [780.676469] remove   /devices/css0/0.0.0005/0.0.0005/virtio5/block/vdc (block)
UDEV  [780.678185] remove   /devices/virtual/bdi/252:32 (bdi)
KERNEL[780.708055] unbind   /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
KERNEL[780.708078] remove   /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
KERNEL[780.708088] unbind   /devices/css0/0.0.0005/0.0.0005 (ccw)
KERNEL[780.708101] remove   /devices/css0/0.0.0005/0.0.0005 (ccw)
KERNEL[780.708109] unbind   /devices/css0/0.0.0005 (css)
KERNEL[780.708117] remove   /devices/css0/0.0.0005 (css)
UDEV  [780.708779] unbind   /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
UDEV  [780.709099] remove   /devices/css0/0.0.0005/0.0.0005/virtio5 (virtio)
UDEV  [780.709397] unbind   /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [780.709670] remove   /devices/css0/0.0.0005/0.0.0005 (ccw)
UDEV  [780.709971] unbind   /devices/css0/0.0.0005 (css)
UDEV  [780.710194] remove   /devices/css0/0.0.0005 (css)

The events on attach are exactly the same, but in slightly different order.
On detach no events are fired at all - so I guess userspace isn't guilty of
not doing anything if there is no event?
As far as i am right now I'd point to the guest kernel, let me try a few other
kernels ...