misc/coverity: Model __builtin_unreachable()

This resolves 23 issues Coverity had identified by following the false path of
an ASSERT().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

misc/coverity/model.c

@@ -120,6 +120,16 @@ void libxl__ctx_unlock(libxl_ctx *ctx)
     __coverity_recursive_lock_release__(&ctx->lock);
 }
 
+/*
+ * Coverity doesn't understand __builtin_unreachable(), which causes it to
+ * incorrectly find issues based on continuing execution along the false
+ * branch of an ASSERT().
+ */
+void __builtin_unreachable(void)
+{
+    __coverity_panic__();
+}
+
 /*
  * Local variables:
  * mode: C