dom0: block-snapshot: add prepare and cleanup actions
dom0: block-snapshot: add prepare and cleanup actions

"prepare" and "cleanup" actions can be used to setup device manually - not from
udev. This is used by qvm-revert-template-changes.

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