systemd: do not fail qubes-vm@ service when the VM is already running
systemd: do not fail qubes-vm@ service when the VM is already running

Fixes QubesOS/qubes-issues#1528

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