conf: modify tracking of encrypted images

conf: modify tracking of encrypted images

A future patch will merge virStorageFileMetadata and virStorageSource,
but I found it easier to do if both structs use the same information
for tracking whether a source file needs encryption keys.

* src/util/virstoragefile.h (_virStorageFileMetadata): Prepare
full encryption struct instead of just a bool.
* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Use transfer semantics.
* src/storage/storage_backend_gluster.c
(virStorageBackendGlusterRefreshVol): Likewise.
* src/util/virstoragefile.c (virStorageFileGetMetadataInternal):
Populate struct.
(virStorageFileFreeMetadata): Adjust clients.
* tests/virstoragetest.c (testStorageChain): Likewise.

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