libblkid: ignore private LVM devices

The virtual private LVM devices do not contain any blkid relevant data
and it does not make any sense to scan for superblocks or partitions
on the devices, because we can interpret data from the devices in bad
way.

Unfortunately, for LVM has "private" very special meaning. The private
LVM devices are accessible and readable (according to LVM guys it's
feature, because debugging etc.).

The problem is pretty visible with LVM thin provisioning where a virtual
pool device contains segments from the top-level thin devices.

Mountable top-level LVM-thin device:

	# blkid -o udev -p /dev/mapper/vg-thin1
	ID_FS_UUID=57ed6490-903b-416c-91d2-6d06804ec60c
	ID_FS_TYPE=ext4

Virtual private LVM-pool device (contains data from all thin devices):

	# blkid -o udev -p /dev/mapper/vg-pool0
	ID_FS_UUID=57ed6490-903b-416c-91d2-6d06804ec60c
	ID_FS_TYPE=ext4

... this is incorrect, vg-pool0 is unmountable.

LVM uses special UUID suffixes for private devices. All devices with
uuid in format "LVM-<uuid>-<type>" are private.

This patch modifies libblkid to not scan such devices. The high-level
API ignores such devices at all now.

The low-level API allows to assign the device to blkid_prober, but all
scan functions always return nothing and library does not read
anything from the device. The another functionality (get parental
device, topology, sector sizes, etc.) still works as expected. The
change affects only probing code.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1101345
Signed-off-by: Karel Zak <kzak@redhat.com>