core dom0 rpm: restart qubes_netvm only when using netvm in Dom0
core dom0 rpm: restart qubes_netvm only when using netvm in Dom0

If we use a separate netvm, then core update in Dom0 doesn't really change the networking,
worse, if we restarted netvm it would get another XID, which would break our DispVM savefile.

One day we should fix it!

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