Refactor virStorageFileGetMetadataFromFD to separate functionality

Refactor virStorageFileGetMetadataFromFD to separate functionality

The virStorageFileGetMetadataFromFD did two jobs in one. First
it probed for storage type, then it extracted metadata for the
type. It is desirable to be able to separate these jobs, allowing
probing without querying metadata, and querying metadata without
probing.

To prepare for this, split out probing code into a new pair of
methods

  virStorageFileProbeFormatFromFD
  virStorageFileProbeFormat

* src/util/storage_file.c, src/util/storage_file.h,
  src/libvirt_private.syms: Introduce virStorageFileProbeFormat
  and virStorageFileProbeFormatFromFD