storage: Add param to check whether we can write a disk label

storage: Add param to check whether we can write a disk label

Modify virStorageBackendDiskValidLabel to add a 'writelabel' parameter.
While initially for the purpose of determining whether the label should
be written during DiskBuild, a future use during DiskStart could determine
whether the pool should be started using the label found. Augment the
error messages also to give a hint as to what someone may need to do
or why the command failed.