When you import a quickstart containing a jboss-service.xml file, you get the following XML:

cvc-complex-type.2.3: Element 'attribute' cannot have character [children], because the type's content type is element-only.

The quickstart source is located here:
https://github.com/jboss-developer/jboss-eap-quickstarts/tree/6.4.x/helloworld-mbean

I searched the https://github.com/jbossas/jboss-eap/ source code for other jboss-service.xml examples, and found a few with syntax similar to what the quickstarts use:

• sar/src/test/resources/test/serviceXmlDeployment.jar/META-INF/jboss-service.xml
• testsuite/integration/basic/src/test/java/org/jboss/as/test/integration/sar/injection/jboss-service.xml
• testsuite/integration/smoke/src/test/java/org/jboss/as/test/smoke/sar/jboss-service.xml
• arquillian/container-managed/src/test/resources/sar-example.sar/META-INF/jboss-service.xml
• arquillian/container-remote/src/test/resources/sar-example.sar/META-INF/jboss-service.xml

There seems to be an error in the schema:
http://www.jboss.org/schema/jbossas/jboss-service_7_0.xsd

The quickstart bug is located here: https://bugzilla.redhat.com/show_bug.cgi?id=1178782