init/FreeBSD: fix xencommons so it can only be launched by Dom0

init/FreeBSD: fix xencommons so it can only be launched by Dom0

At the moment the execution of xencommons is gated on the presence of the
privcmd device, but that's not correct, since privcmd is available to all Xen
domains (privileged or unprivileged). Instead of using privcmd use the
xenstored device, which will only be available to the domain that's in charge
of running xenstored, and thus xencommons.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
(cherry picked from commit c875b9778da0c56a0c118626771465b87df31fe8)