vm: Fix open() result checking
vm: Fix open() result checking

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