fix(virtio): route invalid queue_sel in queue_notify_off to a reserved slot

[Lencerf]

When a driver selects an out-of-bounds queue index (queue_sel >= num_queues) and reads LAYOUT_QUEUE_NOTIFY_OFF, returning queue_sel as-is causes the driver to calculate a notify address past the mapped BAR memory, resulting in an undefined behavior.

Allocate an additional 4-byte slot at the end of the notify capability and MMIO region, and return min(queue_sel, num_queues) for LAYOUT_QUEUE_NOTIFY_OFF. This routes invalid queue notifications to the reserved slot, which is safely ignored by the MMIO handler without accessing unmapped memory or falsely triggering valid queues.

[coveralls]

Coverage Report for CI Build 32947717444

Coverage increased (+0.2%) to 28.142%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: 6 uncovered changes across 1 file (2 of 8 lines covered, 25.0%).
• 4 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
alioth/src/virtio/pci.rs	8	2	25.0%

Coverage Regressions

4 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
alioth/src/virtio/dev/vsock/uds_vsock.rs	2	83.13%
alioth/src/virtio/pci.rs	2	15.86%

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Coverage Stats

Relevant Lines:	14826
Covered Lines:	4047
Line Coverage:	27.3%
Relevant Branches:	1342
Covered Branches:	503
Branch Coverage:	37.48%
Branches in Coverage %:	Yes
Coverage Strength:	39.86 hits per line

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💛 - Coveralls