Allow probing of image formats without version information

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: Adam Litke <agl@us.ibm.com>