conf: delete useless backingStoreIsFile field
    
    Finally starting to prune away some of the old fields that have
    been made redundant by the new fields, on my way towards directly
    reusing virStorageSource.
    
    * src/util/virstoragefile.h (_virStorageFileMetadata): Drop
    field.
    * src/util/virstoragefile.c (virStorageFileGetMetadataInternal)
    (virStorageFileChainLookup): Adjust callers.
    * tests/virstoragetest.c (_testFileData, testStorageChain)
    (mymain): Simplify test.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>