storage: Refactor disk label checking
storage: Refactor disk label checking

Create a new function virStorageBackendDiskValidLabel to handle checking
whether there is a label on the device and whether it's valid or not.
While initially for the purpose of determining whether the label can be
overwritten during DiskBuild, a future use during DiskStart could determine
whether the pool should be started using the label found.

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