tools/hotplug: update systemd dependency to use service instead of socket

tools/hotplug: update systemd dependency to use service instead of socket

Since commit 4542ae340d75bd6319e3fcd94e6c9336e210aeef ("tools/hotplug:
systemd xenstored dependencies") all service files use the .socket unit
as startup dependency. While this happens to work for boot it fails for
shutdown because a .socket does not seem to enforce ordering. When
xendomains.service runs during shutdown then systemd will stop
xenstored.service at the same time.

Change all "xenstored.socket" to "xenstored.service" to let systemd know
that xenstored has to be shutdown after everything else.

Reported-by: Mark Pryor <tlviewer@yahoo.com>
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>