libblkid: ignore empty MBR on LVM device

libblkid: ignore empty MBR on LVM device

It's possible to use boot sector and empty MBR on LVM physical volume
to make LVM disk bootable. In this case MBR should be ignored and disk
reported as LVM.

Just for the record, this is ugly non-default LVM setup maintained for
backward compatibility (yes, LVM guys don't like it too).

Unfortunately people still use it. The proper way is to use regular
partitioned disk.

Reported-by: Xen <list@xenhideout.nl>
Signed-off-by: Karel Zak <kzak@redhat.com>