Initialize vnet.ram unconditionally for device tree

When no network device is specified via -n option, the virtio-net device
was left uninitialized but still exposed to guest via device tree. This
caused segmentation fault when guest attempted to initialize the device.

Root cause analysis:
- WFI merge (e4ae87e) introduced conditional initialization:
  if (netdev) { virtio_net_init(); vnet.ram = ram; }
- Previous code always set vnet.ram regardless of netdev
- Guest kernel initializes all devices in device tree
- Without vnet.ram pointer, QueueReady handler crashed accessing NULL

Author: jserv

main.c

@@ -774,12 +774,15 @@ static int semu_init(emu_state_t *emu, int argc, char **argv)
     emu->uart.in_fd = 0, emu->uart.out_fd = 1;
     capture_keyboard_input(); /* set up uart */
 #if SEMU_HAS(VIRTIONET)
+    /* Always set ram pointer, even if netdev is not configured.
+     * Device tree may still expose the device to guest.
+     */
+    emu->vnet.ram = emu->ram;
     if (netdev) {
         if (!virtio_net_init(&emu->vnet, netdev)) {
             fprintf(stderr, "Failed to initialize virtio-net device.\n");
             return 1;
         }
-        emu->vnet.ram = emu->ram;
         netdev_ready = true;
     }
 #endif

virtio-net.c

@@ -359,6 +359,10 @@ void virtio_net_refresh_queue(virtio_net_state_t *vnet)
         (vnet->Status & VIRTIO_STATUS__DEVICE_NEEDS_RESET))
         return;
 
+    /* Skip if peer network device is not initialized */
+    if (!vnet->peer.op)
+        return;
+
     netdev_impl_t dev_type = vnet->peer.type;
 #define _(dev) NETDEV_IMPL_##dev
     switch (dev_type) {