core: fix creation of private.img at VM startup
core: fix creation of private.img at VM startup

This code is used when VM is migrated from older system, where HVM
didn't have private.img.

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