Commit fff74b27 moved the probing into virStorageFileGetMetadataFromBuf
but didn't update the format in volume definition.

https://bugzilla.redhat.com/show_bug.cgi?id=1104908
(cherry picked from commit c110cdb2)

Conflicts:
	src/storage/storage_backend_fs.c
      I didn't backport the refactors - febcb89c and effbb11e