Fix disk direct lun attrs

@ohadlevy about the "why" :) - not directly related to this patch, but it's a good place to explain.
There are different types of disks in oVirt, and while the general structure and main attributes are the same and always returned, some attributes differ based on a use case - for an image and a direct lun disk there can be attr existing in the first, and non existing in the second and vise versa.
For some attributes it's less important as they are not consumed by rbovirt and hence do not crash because they are not read from the response xml.
For the ones that are read from the xml in rbovirt, the attempt to read a non existing attribute and invoke a method on it obviously crashes.
For example, quota attr exists only sometimes, it's not mandatory. 
In direct lun disk - such as in this particular case, there is no storage domain to which the disk belongs because it is a direct lun disk, hence, any attempt to read a storage_domain section in the xml and the attempt to extract a domain id will fail because it's not present in the xml. Same about sparse and format properties.