fix(loongarch64): Fix Virtio using too much CPU

[weifenjihe]

Summary

Fix excessive hvisor-virtio CPU usage on LoongArch64.

• Enable epoll_wait() instead of continuous polling.
• Wake userspace through eventfd.
• Acknowledge IRQ 4 (HWI2) after each wakeup.

Tested by building hvisor-tool and hvisor.ko for LoongArch64.

[li041]

Thanks for your contribution. This update unifies the LoongArch64 request-handling path with the other architectures: it moves LoongArch64 from the previous polling loop to the modern epoll_wait() event-driven interface, and drains pending requests before entering the wait. This reduces unnecessary CPU usage and keeps the backend behavior more consistent across architectures.

One small follow-up: the current CI code style check is failing on the changed code. Please run clang-format (version 14 used by CI) on driver/hvisor_main.c, include/hvisor.h, and tools/virtio/virtio.c before merging.

Thanks again for the contribution!

[li041]

LGTM!