core: fix handling vm.start_time for just shutdown VM
core: fix handling vm.start_time for just shutdown VM

That xenstore entry may be already removed even when libvirt still
reports the VM as running.

Fix QubesOS/qubes-issues#2127

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