替换文本，尾部属性。

import lxml.etree as ET

with open('1.xml', 'rb+') as f:
    tree = ET.parse(f)
    root = tree.getroot()
    for elem in root.getiterator():
        if elem.text:
            elem.text = elem.text.replace('VirtualBox', 'Xen')
        if elem.tail:
            elem.tail = elem.tail.replace('VirtualBox', 'Xen')

    f.seek(0)
    f.write(ET.tostring(tree, encoding='UTF-8', xml_declaration=True))
    f.truncate()