Don't validata filesystem target type
    
    When using QEMU's 9pfs the target "dir" element is not necessarily an
    absolute path but merely an arbitrary identifier. So validation in that
    case currently fails with the misleading
    
       $ virt-xml-validate /tmp/test.xml
       Relax-NG validity error : Extra element devices in interleave
       /tmp/test.xml:24: element devices: Relax-NG validity error : Element domain failed to validate content
       /tmp/test.xml fails to validate