storage/lvm: fix logic regarding snapshots, start, stop etc
storage/lvm: fix logic regarding snapshots, start, stop etc

There are mutiple cases when snapshots are inconsistently created, for
example:
 - "-back" snapshot created from the "new" data, instead of old one
 - "-snap" created even when volume.snap_on_start=False
 - probably more

Fix this by following volume.snap_on_start and volume.save_on_stop
directly, instead of using abstraction of old volume types.

QubesOS/qubes-issues#2256

QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin