virtionet: Add back legacy IRQ support

Author: fwsGonzo

src/drivers/virtionet.cpp

@@ -156,7 +156,8 @@ VirtioNet::VirtioNet(hw::PCI_Device& d)
   }
   else
   {
-    assert(0 && "Legacy IRQs not supported");
+    auto irq = Virtio::get_legacy_irq();
+    IRQ_manager::get().subscribe(irq, {this, &VirtioNet::legacy_handler});
   }
 
 #ifndef NO_DEFERRED_KICK
@@ -243,6 +244,12 @@ void VirtioNet::msix_xmit_handler()
   }
 }
 
+void VirtioNet::legacy_handler()
+{
+  msix_recv_handler();
+  msix_xmit_handler();
+}
+
 void VirtioNet::add_receive_buffer(uint8_t* pkt)
 {
   // offset pointer to virtionet header

src/drivers/virtionet.hpp

@@ -220,6 +220,8 @@ class VirtioNet : Virtio, public net::Link_layer<net::Ethernet> {
   void msix_xmit_handler();
   void msix_conf_handler();
 
+  void legacy_handler();
+
   /** Allocate and queue buffer from bufstore_ in RX queue. */
   void add_receive_buffer(uint8_t*);