qubes-restore: ensure we don't read outside the buffer
qubes-restore: ensure we don't read outside the buffer

Marek says the 20 offset was there for a previous Xen
save file format, and is no longer needed.

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