Use constants for PE size bounds
Add tests for the no-device LVM functions

Some functions from the LVM plugin don't operate on any device and should thus
be put into a separate TestCase.
Fix typo in the doc text
Give temp sparse files better names and always remove them
Device should only be used if updating cache in pvscan

When not updating the cache, the pvscan command doesn't take the device
argument. We should reflect that behaviour.
First of the LVM tests requiring devices