conf: Allow all volume modes for disk type='lun' sources
conf: Allow all volume modes for disk type='lun' sources

Commit 82ba41108acd0f3f made possible to use direct mapped iSCSI
volumes in qemu as disk sources but didn't remove the define time check.

Rework the check by simplifying the condition and allow any volumes to
be used with disk type='lun'.

zippy2/libvirt,zippy2/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,taget/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,taget/libvirt,crobinso/libvirt,jfehlig/libvirt,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,datto/libvirt,taget/libvirt,VenkatDatta/libvirt,zippy2/libvirt,eskultety/libvirt,taget/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,eskultety/libvirt,crobinso/libvirt,libvirt/libvirt,eskultety/libvirt,andreabolognani/libvirt,taget/libvirt,andreabolognani/libvirt,datto/libvirt,datto/libvirt,olafhering/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,datto/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,eskultety/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,crobinso/libvirt,jfehlig/libvirt,datto/libvirt,zippy2/libvirt