storage: backend: Use correct stringifier for pool type
storage: backend: Use correct stringifier for pool type

When registering a storage poll backend, the code would use
virStorageTypeToString instead of virStoragePoolTypeToString. The
following message would be logged:

virDriverLoadModuleFunc:71 : Lookup function 'virStorageBackendSCSIRegister'
virStorageBackendRegister:174 : Registering storage backend '(null)'

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