conf: Validate disk lun using correct types

conf: Validate disk lun using correct types

https://bugzilla.redhat.com/show_bug.cgi?id=1201143

The formatdomain.html description for <disk> device 'lun' indicates that
it must be either a type 'block' or type 'network' with protocol 'iscsi';
however, we did not make that check until domain startup.

This caused issues for virt-manager which had an unexpected failure at
run time rather config time.

This patch adds a check in post part disk device checking for the specific
and supported lun types as well as adjusting the test failure to be for
parse config rather than run time.