cpu: Fix Minor drain issues when switched out

The Minor CPU currently doesn't drain properly when it is switched
out. This happens because Fetch 1 expects to be in the FetchHalted
state when it is drained. However, because the CPU is switched out, it
is stuck in the FetchWaitingForPC state. Fix this by ignoring drain
requests and returning DrainState::Drained from MinorCPU::drain() if
the CPU is switched out. This is always safe since a switched out CPU,
by definition, doesn't have any instructions in flight.

src/cpu/minor/cpu.cc

@@ -201,6 +201,11 @@ MinorCPU::startup()
 DrainState
 MinorCPU::drain()
 {
+    if (switchedOut()) {
+        DPRINTF(Drain, "Minor CPU switched out, draining not needed.\n");
+        return DrainState::Drained;
+    }
+
     DPRINTF(Drain, "MinorCPU drain\n");
 
     /* Need to suspend all threads and wait for Execute to idle.