virtio-net-device (MMIO) not handling network interrupts

[mkroening]

The feature was introduced in #296.

To reproduce, run in hermit-rs:

export HERMIT_LOG_LEVEL_FILTER=Debug
export HERMIT_IP=192.168.76.9
export HERMIT_GATEWAY=192.168.76.2

cargo build -Zbuild-std=std,panic_abort --target=x86_64-unknown-hermit --package httpd --no-default-features --features hermit/tcp

qemu-system-x86_64 \
    -display none -serial stdio -kernel kernel/hermit-loader-x86_64 \
    -M microvm,x-option-roms=off,pit=off,pic=off,rtc=on,auto-kernel-cmdline=off,acpi=off -global virtio-mmio.force-legacy=off -nodefaults -no-user-config \
    -cpu Skylake-Client -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -initrd target/x86_64-unknown-hermit/debug/httpd \
    -smp 1 -m 2G -netdev user,id=u1,hostfwd=tcp::9975-:9975,hostfwd=udp::9975-:9975,net=192.168.76.0/24,dhcpstart=192.168.76.9 -device virtio-net-device,netdev=u1 \
    -append "-freq 3000" \
    -object filter-dump,id=f1,netdev=u1,file=dump.pcap

To reproduce with the original code:

git clone git@github.com:hermit-os/hermit-rs.git hermit-mmio
cd hermit-mmio
git checkout hermit-sys-0.1.25
git submodule update --init

cd loader
make
cd ..

cd libhermit-rs
cargo update --package log --precise 0.4.0
cargo update --package crossbeam-utils --precise 0.8.7

# Change MMIO constants in kernel:
# pub const MMIO_START: usize = 0x00000000feb00000;
# pub const MMIO_END: usize = 0x00000000feb0ffff;
# const IRQ_NUMBER: u32 = 23;

cd ..

cargo build --no-default-features --features smoltcp -p httpd

qemu-system-x86_64 \
    -display none -serial stdio -kernel loader/target/x86_64-unknown-hermit-loader/debug/rusty-loader \
    -M microvm,x-option-roms=off,pit=off,pic=off,rtc=on,auto-kernel-cmdline=off,acpi=off -global virtio-mmio.force-legacy=off -nodefaults -no-user-config \
    -cpu Skylake-Client -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -initrd target/x86_64-unknown-hermit/debug/httpd \
    -smp 1 -m 2G -netdev user,id=u1,hostfwd=tcp::9975-:9975,hostfwd=udp::9975-:9975,net=192.168.76.0/24,dhcpstart=192.168.76.9 -device virtio-net-device,netdev=u1 \
    -append "-freq 3000" \
    -object filter-dump,id=f1,netdev=u1,file=dump.pcap