xen init script: move init_dom0 into xenstored start

xen init script: move init_dom0 into xenstored start

Executing this is only necessary after starting xenstored. In all other
cases it just prints noop spam to stderr.

    -# /usr/lib/xen-4.11/bin/xen-init-dom0 >/dev/null
    Dom0 is already set up

I don't want to silence stderr, so just move the code into the end of
the xenstored start function.

Also, add a hint that the compatibility if/else part should be removed
after the Buster release.

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>