Allow probing of image formats without version information

Disk image formats that wish to opt-out of version validation are supposed to
set versionOffset to -1 in their fileTypeInfo entry.

By unconditionally returning False for these formats,
virStorageFileMatchesVersion() incorrectly reports a version mismatch when the
test was actually skipped.  The correct behavior is to return True so these
formats can be successfully probed using the magic bytes alone.
Signed-off-by: NAdam Litke <agl@us.ibm.com>

src/util/storage_file.c

@@ -479,7 +479,7 @@ virStorageFileMatchesVersion(int format,
 
     /* Validate version number info */
     if (fileTypeInfo[format].versionOffset == -1)
-        return false;
+        return true;
 
     if ((fileTypeInfo[format].versionOffset + 4) > buflen)
         return false;