upload/koji: extract processXMLRPCResponse method

Fedora 33 ships kolo/xmlrpc with a different API. This commit extracts the
affected code so we can use build flags in the future allowing us to use
both API versions.

internal/upload/koji/koji.go

@@ -315,19 +315,14 @@ func (k *Koji) uploadChunk(chunk []byte, filepath, filename string, offset uint6
 		return err
 	}
 
-	resp := xmlrpc.NewResponse(body)
-	if resp.Failed() {
-		return resp.Err()
-	}
-
 	var reply struct {
 		Size      int    `xmlrpc:"size"`
 		HexDigest string `xmlrpc:"hexdigest"`
 	}
 
-	err = resp.Unmarshal(&reply)
+	err = processXMLRPCResponse(body, &reply)
 	if err != nil {
-		return fmt.Errorf("cannot unmarshal the xmlrpc response: %v", err)
+		return err
 	}
 
 	if reply.Size != len(chunk) {

internal/upload/koji/xmlrpc-response-oldapi.go

@@ -0,0 +1,31 @@
+// This files provides a wrapper around kolo/xmlrpc response handling.
+//
+// Commit e3ad6d89 of the xmlrpc library changed the API of response handling.
+// This means that different APIs are available in Fedora 32 and 33 (it does
+// not matter for RHEL as uses vendored libraries).
+// This wrapper allows us to use both xmlrpc's APIs using buildflags.
+//
+// This file is a wrapper for xmlrpc older than e3ad6d89.
+
+package koji
+
+import (
+	"fmt"
+
+	"github.com/kolo/xmlrpc"
+)
+
+// processXMLRPCResponse is a wrapper around kolo/xmlrpc
+func processXMLRPCResponse(body []byte, reply interface{}) error {
+	resp := xmlrpc.NewResponse(body)
+	if resp.Failed() {
+		return fmt.Errorf("xmlrpc server returned an error: %v", resp.Err())
+	}
+
+	err := resp.Unmarshal(reply)
+	if err != nil {
+		return fmt.Errorf("cannot unmarshal the xmlrpc response: %v", err)
+	}
+
+	return nil
+}