systemd: use standard dependencies for xendriverdomain.service

systemd: use standard dependencies for xendriverdomain.service

Having DefaultDependencies=no means it can be started before / is
remounted read-write, which will result in various failures (to start
with opening the log).
Since "libxl: trigger attach events for devices attached before xl devd
startup" it is no longer important to start it as early as possible,
because it will process devices created before its startup.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>