conf: use common struct in storage volumes

conf: use common struct in storage volumes

A fairly smooth transition.  And now that domain disks and
storage volumes share a common struct, it opens the doors for
a future patch to expose more details in the XML for both
objects.

* src/conf/storage_conf.h (_virStorageVolTarget): Delete.
(_virStorageVolDef): Use common type.
* src/conf/storage_conf.c (virStorageVolDefFree)
(virStorageVolTargetDefFormat): Update clients.
* src/storage/storage_backend.h: Likewise.
* src/storage/storage_backend.c
(virStorageBackendDetectBlockVolFormatFD)
(virStorageBackendUpdateVolTargetInfo)
(virStorageBackendUpdateVolTargetInfoFD): Likewise.
* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>