xen: Set max SCHEDOP_shutdown events to 3

In some instances, the guest may run in an infinite crash loop causing
the entire debug ring buffer to be filled with junk information. We now
limit this to 3 events, allowing us to view useful information.

vmm/include/xen/vcpu.h

@@ -161,6 +161,8 @@ class xen_vcpu {
     uint32_t m_apicid{};
     uint32_t m_acpiid{};
 
+    uint32_t m_max_shutdown_events{};
+
 public:
     ~xen_vcpu() = default;
     xen_vcpu(xen_vcpu &&) = delete;

vmm/src/xen/vcpu.cpp

@@ -656,7 +656,13 @@ bool xen_vcpu::handle_sched_op()
          * see how we got here.
          */
         m_uv_vcpu->set_rax(-EINVAL);
-        return true;
+
+        if (++m_max_shutdown_events == 3) {
+            return false;
+        }
+        else {
+            return true;
+        }
     }
     default:
         printv("%s: cmd=%lu unhandled\n", __func__, cmd);