qubes/vm: fix setting autostart property
qubes/vm: fix setting autostart property

This is actually workaround for systemd bug reported here:
https://bugzilla.redhat.com/show_bug.cgi?id=1181922

qubesos/qubes-issues#925

This is migration of 9bfcb72722170f7410cce1e4a2ab15ee3df11c89 commit to
core3.

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