qubes/ext/pci: fix handling dom0 before starting first VM
qubes/ext/pci: fix handling dom0 before starting first VM

Before starting fist VM, backend/pci xenstore directory does not exists.
Do not crash on it

QubesOS/qubes-issues#2257

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