conf: disk: Extract disk type and device right away
conf: disk: Extract disk type and device right away

Additionally switch to using a common temp variable for the xml
elements.

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