storage: add network-dir as new storage volume type
    
    In the 'directory' and 'netfs' storage pools, a user can see
    both 'file' and 'dir' storage volume types, to know when they
    can descend into a subdirectory.  But in a network-based storage
    pool, such as the upcoming 'gluster' pool, we use 'network'
    instead of 'file', and did not have any counterpart for a
    directory until this patch.  Adding a new volume type
    'network-dir' is better than reusing 'dir', because it makes
    it clear that the only way to access 'network' volumes within
    that container is through the network mounting (leaving 'dir'
    for something accessible in the local file system).
    
    * include/libvirt/libvirt.h.in (virStorageVolType): Expand enum.
    * docs/formatstorage.html.in: Document it.
    * docs/schemasa/storagevol.rng (vol): Allow new value.
    * src/conf/storage_conf.c (virStorageVol): Use new value.
    * src/qemu/qemu_command.c (qemuBuildVolumeString): Fix client.
    * src/qemu/qemu_conf.c (qemuTranslateDiskSourcePool): Likewise.
    * tools/virsh-volume.c (vshVolumeTypeToString): Likewise.
    * src/storage/storage_backend_fs.c
    (virStorageBackendFileSystemVolDelete): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>