Some preparatory work before consolidating storage volume
structs with the rest of virstoragefile.  Making these
changes allows a volume target to be much closer to (a
subset of) the virStorageSource struct.

Making perms be a pointer allows it to be optional if we
have a storage pool that doesn't expose permissions in a
way we can access.  It also allows future patches to
optionally expose permissions details learned about a disk
image via domain <disk> listings, rather than just
limiting it to storage volume listings.

Disk partition types was only used by internal code to
control what type of partition to create when carving up
an MS-DOS partition table storage pool (and is not used
for GPT partition tables or other storage pools).  It was
not exposed in volume XML, and as it is more closely
related to extent information of the overall block device
than it is to the <target> information describing the host
file.  Besides, if we ever decide to expose it in XML down
the road, we can move it back as needed.

* src/conf/storage_conf.h (_virStorageVolTarget): Change perms to
pointer, enhance comments.  Move partition type...
(_virStorageVolSource): ...here.
* src/conf/storage_conf.c (virStorageVolDefFree)
(virStorageVolDefParseXML, virStorageVolTargetDefFormat): Update
clients.
* src/storage/storage_backend_fs.c (createFileDir): Likewise.
* src/storage/storage_backend.c (virStorageBackendCreateBlockFrom)
(virStorageBackendCreateRaw, virStorageBackendCreateExecCommand)
(virStorageBackendUpdateVolTargetInfoFD): Likewise.
* src/storage/storage_backend_logical.c
(virStorageBackendLogicalCreateVol): Likewise.
* src/storage/storage_backend_disk.c
(virStorageBackendDiskMakeDataVol)
(virStorageBackendDiskPartTypeToCreate): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>