dispvm: create /qubes-restore-complete qubesdb key
dispvm: create /qubes-restore-complete qubesdb key

It is used by just started DispVM to notice when restore process
completed. Alternatively it could watch its own domid, but lets do it in
Xen-independent way.

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