Replace signal-based Virtio event notification with eventfd

[agicy]

Background & Problem

Currently, hvisor uses a custom signal (SIGHVI) to notify user space about Virtio interrupts. This approach suffers from high kernel overhead due to context switching, limited scalability, and poor compatibility with modern asynchronous I/O models.

Proposed Solution

Replace the signal mechanism with eventfd. This provides a lightweight, file-descriptor-based notification interface.
Crucially, this change is a prerequisite for future integration with io_uring, which will allow for high-performance asynchronous I/O.

Implementation Details

Kernel Space (driver/hvisor.c):

• Remove SIGHVI generation and task tracking.
• Implement eventfd context management.
• Modify virtio_irq_handler to write to the eventfd counter instead of sending signals.
• Update ioctl to initialize and bind eventfd for user processes.

User Space (tools/virtio.c):

• Remove sigwait and signal handling logic.
• Obtain the eventfd via ioctl.
• Update the main loop to wait on the eventfd (currently via blocking read, preparing the logic for future io_uring submission).
• Process Virtio queues upon eventfd readability.

Expected Outcomes

• Performance: Reduced notification latency and system overhead.
• Architecture: Decoupled signal logic from driver code.
• Future-proofing: Ready for seamless migration to io_uring.