Symfony version(s) affected: 5.0

Description
If you start an XMLEncode::encode process in the middle of a normalization (you need to create an base64 encoded XML sub document), cause an "Wrong Document Error" DomDocument exception at:

Here is the call stack:

• The app call the serialize with: $serializer->serialize($data, "xml", [...context...]);
• The first serialize call the createDomDocument at:
  

  symfony/src/Symfony/Component/Serializer/Encoder/XmlEncoder.php

  Line 93 in 3e57f1f

  $this->dom = $this->createDomDocument($context);

• The XMLEncoder call normalizer at:
  

  symfony/src/Symfony/Component/Serializer/Encoder/XmlEncoder.php

  Line 415 in 3e57f1f

  $data = $this->serializer->normalize($data, $this->format, $this->context);

• In CustomNormalizer::normalize we call
  $serializer->serialize($otherdata, "xml", [...other context...]);
• Second serialize override dom at:
  

  symfony/src/Symfony/Component/Serializer/Encoder/XmlEncoder.php

  Line 93 in 3e57f1f

  $this->dom = $this->createDomDocument($context);

• When we done the normalization the encoder fails here because the second serializer did override the dom:
  

  symfony/src/Symfony/Component/Serializer/Encoder/XmlEncoder.php

  Line 448 in 3e57f1f

  $parentNode->appendChild($node);

How to reproduce
Create an XML with an base64 encoded xml subcontent:

<?xml version="1.0"?>
<node>...base64encoded xml...</node>

xml to base64 encode:

<?xml version="1.0"?>
<node2>test</node2>

Possible Solution
DOMDocument of XMLEncoder should stackable.

Additional context
I will post repo, test and PR later.