"default-virtual-server" is a mandatory attribute as per the domain.xml DTD, but that should not prevent us from supporting an empty string as its value in order to disable the concept:
<http-listener [...] default-virtual-server=""/>
This would be trivial to implement.
As for the purpose of the "hosts" attribute in virtual-server, it lists all the "Host" request headers that will be mapped to the virtual-server. This makes it possible to test virtual hosting without having to make any changes to DNS. For instance, if you have a virtual-server with a "hosts" attribute equal to "myhost", the following request would be mapped to it:
GET /abc/def HTTP/1.1
Host: myhost
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