tools/xenconsoled: Increase file descriptor limit

tools/xenconsoled: Increase file descriptor limit

XenServer's VM density testing uncovered a regression when moving from
sysvinit to systemd where the file descriptor limit dropped from 4096 to
1024. (XenServer had previously inserted a ulimit statement into its
initscripts.)

One solution is to use LimitNOFILE=4096 in xenconsoled.service to match the
lost ulimit, but that is only a stopgap solution.

As Xenconsoled genuinely needs a large number of file descriptors if a large
number of domains are running, attempt to increase the limit.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
(cherry picked from commit 588df84c0d702e835e526ecef3af6c5444857558)
(cherry picked from commit 9d5b2b01024d18b2135c1b0deebb8bfbf5dced5f)
(cherry picked from commit a490f8df8c989483624ebd6befb63377be55722b)