Starting with Linux 4.18, there is new device node for issuing
hypercalls from user space - /dev/xen/hypercall (partially duplicating
functionality of /dev/xen/privcmd). New Xen tools (4.12) make use of it,
so make it available for them. Otherwise such tools will fail the
operation (there is no fallback to privcmd on EACCESS).
Recently we've switched all xenstore access to the new interface
(instead of deprecated /proc/xen/xenbus). Mostly because of deadlock in
/proc/xen/xenbus implementation.