Implement short expose syntax for XML as it is available for YAML

[goetas]

Q	A
Bug fix?	no
New feature?	yes
Doc updated	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-
License	MIT

When specifying exclusion-policy=ALL, mentioning a property makes it implicitly exposed

<?xml version="1.0" encoding="UTF-8"?>
<serializer>
    <class name="JMS\Serializer\Tests\Fixtures\Person" exclusion-policy="ALL">
        <property name="name"/>
    </class>
</serializer>