systemd: load xen-privcmd module

It is needed for vchan communication. It was loaded implicitly by mount
/proc/xen, but since we're moving away from this legacy interface, load
it explicitly.

QubesOS/qubes-issues#2194

vm-systemd/qubes-core.conf

@@ -3,3 +3,4 @@ xen-blkback
 u2mfn
 xen-gntalloc
 xen-gntdev
+xen-privcmd