refactor(hv): replace trait IoeventFd with struct Notifier

[Lencerf]

Replace the trait IoeventFd with a concrete type struct Notifier, as only KVM supports I/O event notifications via KVM_IOEVENTFD. This simplifies functions and types.

The refactoring is also needed to support vfio-user VFIO_USER_DEVICE_GET_REGION_IO_FDS, as for vio-user devices, the eventfds are created by the device process and passed to the VMM via Unix domain sockets. Then we cannot have the IoeventFd constructor bound to the registry type.

Assisted-by: Antigravity:Claude-Opus-5

[coveralls]

Coverage Report for CI Build 34741906547

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Coverage: 37.328%

Details

• Patch coverage: 130 uncovered changes across 19 files (51 of 181 lines covered, 28.18%).

Uncovered Changes

Top 10 Files by Coverage Impact	Changed	Covered	%
alioth/src/virtio/vu/frontend.rs	13	0	0.0%
alioth/src/virtio/worker/io_uring.rs	12	0	0.0%
alioth/src/virtio/dev/vsock/vhost_vsock.rs	11	0	0.0%
alioth/src/virtio/dev/blk.rs	10	0	0.0%
alioth/src/virtio/dev/fs/vu.rs	10	0	0.0%
alioth/src/virtio/dev/net/tap.rs	10	0	0.0%
alioth/src/virtio/vu/backend.rs	9	0	0.0%
alioth/src/virtio/dev/balloon.rs	8	0	0.0%
alioth/src/virtio/dev/fs/fs.rs	8	0	0.0%
alioth/src/virtio/dev/net/vmnet.rs	8	0	0.0%
Total (20 files)	181	51	28.18%

Coverage Regressions

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Coverage Stats

Relevant Lines:	15379
Covered Lines:	5567
Line Coverage:	36.2%
Relevant Branches:	1552
Covered Branches:	753
Branch Coverage:	48.52%
Branches in Coverage %:	Yes
Coverage Strength:	45.23 hits per line

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