Check for reboot completion and rebalance threads

Rebalance needs to occur after crosvm restarts and after "adb reboot"
occurs in the guest. Check for another BootCompleted event in the
monitoring thread and rebalance the threads.

Bug: b/408290580
Test: m && adb reboot and saw that the threads migrated

Author: daidavid1

base/cvd/cuttlefish/host/commands/run_cvd/boot_state_machine.cc

@@ -511,6 +511,15 @@ class CvdBootStateMachine : public SetupFeature, public KernelLogPipeConsumer {
         if (!read_result.ok()) {
           return;
         }
+        if ((*read_result)->event == monitor::Event::BootCompleted) {
+          LOG(INFO) << "Virtual device rebooted successfully";
+          if (!instance_.vcpu_config_path().empty()) {
+            auto res = WattsonRebalanceThreads(instance_.id());
+            if (!res.ok()) {
+              LOG(ERROR) << res.error().FormatForEnv();
+            }
+          }
+        }
       }
     }
   }