sims/e1000_gem5: schedule dma events as well to match gem5

Previously we were calling completions etc. directly, but then the
prioritization does not match what gem5 does. This is slightly slower, but we
want to match gem5 exactly here.

sims/nic/e1000_gem5/e1000_gem5.cc

@@ -46,10 +46,15 @@ void IGbE::RegWrite(uint8_t bar, uint64_t addr, const void *src,
 void IGbE::DmaComplete(nicbm::DMAOp &op)
 {
     Gem5DMAOp *dma = dynamic_cast <Gem5DMAOp *>(&op);
-    dma->ev_.sched = false;
-    dma->ev_.callback();
-    if (dma->write_)
+    if (dma->write_) {
         delete[] ((uint8_t *) dma->data_);
+    } else {
+        // schedule callback event. THis is at the current time, but can't call
+        // directly to ensure event priorities are respected.
+        dma->ev_.sched = true;
+        dma->ev_.time_ = runner_->TimePs();
+        runner_->EventSchedule(dma->ev_);
+    }
     delete dma;
 }
 
@@ -133,21 +138,18 @@ void IGbE::intrClear()
 void IGbE::dmaWrite(Addr daddr, size_t len, EventFunctionWrapper &ev,
     const void *buf, Tick delay)
 {
-    ev.sched = true;
-
     Gem5DMAOp *op = new Gem5DMAOp(ev);
     op->data_ = new uint8_t[len];
     memcpy(op->data_, buf, len);
     op->len_ = len;
     op->write_ = true;
     op->dma_addr_ = daddr;
+    op->priority_ = 1;
+    op->time_ = runner_->TimePs() + delay;
+    runner_->EventSchedule(*op);
 
-    if (delay == 0) {
-        runner_->IssueDma(*op);
-    } else {
-        op->time_ = runner_->TimePs() + delay;
-        runner_->EventSchedule(*op);
-    }
+    ev.time_ = runner_->TimePs() + delay;
+    runner_->EventSchedule(ev);
 }
 
 void IGbE::dmaRead(Addr saddr, size_t len, EventFunctionWrapper &ev,
@@ -160,13 +162,10 @@ void IGbE::dmaRead(Addr saddr, size_t len, EventFunctionWrapper &ev,
     op->len_ = len;
     op->write_ = false;
     op->dma_addr_ = saddr;
+    op->time_ = runner_->TimePs() + delay;
+    op->priority_ = 2;
+    runner_->EventSchedule(*op);
 
-    if (delay == 0) {
-        runner_->IssueDma(*op);
-    } else {
-        op->time_ = runner_->TimePs() + delay;
-        runner_->EventSchedule(*op);
-    }
 }
 
 bool IGbE::sendPacket(EthPacketPtr p)