Instead of having duplicated code in qemuStorageLimitsRefresh and
virStorageBackendUpdateVolTargetInfo to get capacity specific data
about the storage backing source or volume -- create a common API
to handle the details for both.

As a side effect, virStorageFileProbeFormatFromBuf returns to being
a local/static helper to virstoragefile.c

For the QEMU code - if the probe is done, then the format is saved so
as to avoid future such probes.

For the storage backend code, there is no need to deal with the probe
since we cannot call the new API if target->format == NONE.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>