Add: Allow to convert an XmlCommandElement into bytes

This will allow to transfer an XmlCommandElement and therefore a
XmlCommand over a socket.

gvm/xml.py

@@ -70,11 +70,17 @@ def append_xml_str(self, xml_text: str) -> None:
         self._element.append(node)
 
     def to_string(self) -> str:
-        return xmltostring(self._element).decode("utf-8")
+        return self.to_bytes().decode("utf-8")
+
+    def to_bytes(self) -> bytes:
+        return xmltostring(self._element)
 
     def __str__(self) -> str:
         return self.to_string()
 
+    def __bytes__(self) -> bytes:
+        return self.to_bytes()
+
 
 class XmlCommand(XmlCommandElement):
     def __init__(self, name: str) -> None: