According to docs, using 'lun' as a value for device attribute is only valid
with disk types 'block' and 'network'. However current RNG schema also allows
a combination type='file' device='lun' which results in a successfull
xml validation, but fails at qemuBuildCommandLine.
Besides fixing the RNG schema, this patch also adds a qemuxml2argvtest
for this case.
https://bugzilla.redhat.com/show_bug.cgi?id=1210669
