tools/hotplug: Add native systemd xendriverdomain.service

tools/hotplug: Add native systemd xendriverdomain.service

A dedicated Xen driver domain init service starts "xl devd" in domU. But
currently, it is only supplied in the form of a SysV init script, which
systemd users run through a backward compatiblity wrapper automatically
generated by systemd-sysv-generator. This patch adds a (naturally more
lightweight) native systemd unit to be used instead.

The xendriverdomain service is only relevant to domU, but should not run
in dom0. Therefore, the systemd unit uses "ConditionVirtualization=xen",
which evaluates to true in domU and (since systemd version 214, released
on 2014-06-11) to false in dom0. Users or distributors who need to be
compatible with even older systemd versions, but still want to prevent
"xl devd" startup in dom0, could add the following line in [Service]:
ExecStartPre=/bin/sh -c "! grep -q control_d /proc/xen/capabilities"

(Please rerun autogen.sh after applying this patch)

Signed-off-by: Rusty Bird <rustybird@openmailbox.org>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ wei: rerun autogen.sh ]