Fix max LV size test on 64-bit archs
Fix LVM2 variable's holding PV count name

Otherwise we get nonsense values in VGData.pv_count.
Fix symbol names in some doc texts

If invalid, they cause the param descriptions to be ignored and parameters
being treated wrongly when called via introspection.
Fix option for specifying thin pool in lvm_thsnapshotcreate
Device-based tests for the LVM plugin