configure.ac: Include liblvm2app in configuration summary
lvm: Change unused return value to void
lvm: Ignore virtual volumes

The sum of the sizes reported by a volume group should equal the size
of the volume group.  Virtual volumes do not directly correspond to
allocated storage (and, in fact, may be larger than the entire volume
group), so must be ignored.
lvm: Report used/free space for thin pools and thick snapshots

Thin pools and traditional snapshot volumes fill up as I/O is performed
to their associated thin volumes or origins.  When they run out of free
space, things break, so it's useful to monitor their disk utilization.

Add new plugin instances for thin pool data LVs, thin pool metadata LVs,
and thick snapshot LVs, each with "used" and "free" type instances.
lvm: Fix segfault if a VG contains no LVs
lvm: Avoid lvm_lv_get_attr()

It is only available in liblvm2app >= 2.02.99, which is not in Debian
Wheezy.
configure.ac: Check for newest liblvm2app symbol needed by lvm

lvm_lv_get_property() was added in 2.02.75.
Revert "lvm: Fix segfault if a VG contains no LVs"

Avoid merge conflict with 25d7de9 in 5.4 branch, per #603.

This reverts commit 45805cf888cda0e840d774f1d42cd21525a722ea.
Merge remote-tracking branch 'origin/pr/603'
lvm: Do not segfault when there are no VGs
lvm: change vg_read() return value to void

To match function prototype change from 55cd1f9a and avoid conflict with
bugfix from 25d7de9.