Just ignore the unsupported "sub-LVs" and consider cached LVs normal LVs.

Those that are related to each other should be next to each other and titled.

Internal LVs are not going to be referenced by the DeviceTree instance and are
going to have no parents. Their "parent" LVs are going to be referenced as
'self.parent_lv' instead of 'self.parents[0]' because all the other devices use
the opposite logic for parent-children relations -- children are consisting of
or built on top of parents (whereas "parent" LVs are consisting of their
internal LVs).

This way internal LVs can easily override these methods.

Also make the LVMLogicalVolumeDevice class ready for managing internal LVs.

Instead of calculating static values for number of copies, log size and metadata
size of a given LV, we should use internal LVs assigned to their "parent"
(normal) LVs and calculate such values dynamically from those internal LVs.

These are both valid types for the self.parents attribute.

Thin pools and thin LVs are also LVs so they should be included in the
result. Also don't use the to-be-deprecated-soon getDevicesByType() method.

Thin/cache pools have internal data/metadata LVs the name of which can be
queried. Let's use this functionality to help determining the parent LV if name
matching fails.

Internal metadata LVs of thin pools can be resized so our representation should
also allow it.