qubes/vm: fix handling autostart set/reset
qubes/vm: fix handling autostart set/reset

1. Service symlink must have ".service" extension
2. Disable service on property reset to default

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