Closed issue by xelxebar on void-packages repository

https://github.com/void-linux/void-packages/issues/18676

Description:
### Symptom

With the `xen` package installed, and booting *without* the hypervisor, dmesg gets spammed with the following message once every second:

    [000000.000000] xen_evtchn: unknown parameter 'xen-gnttalloc' ignored

### Cause

The problem can be traced back to the `xenstored` service installed by the `xen` package:

    $ grep xen-gnttalloc /etc/sv/xenstored/run
    modprobe -q xen-evtchn xen-gnttalloc || exit 1

This modprobe fails (logging our error) causing the service to fail. Thus `runit` (futually) tries to restart the service *ad infinitum*, leading to the dmesg spam.

### Potential Solution

The `xenstored` service should gracefully degrade to a noop in the case when xen isn't running. One way we can do this is by checking `xl list`. The following patch to shows a workaround that functions on my machine:

    --- /etc/sv/xenstored/run	2020-01-31 15:03:46.619346755 +0900
    +++ /etc/sv/xenstored/run	2020-01-31 15:03:35.912144255 +0900
    @@ -1,4 +1,5 @@
     #!/bin/sh
    +xl list >/dev/null 2>&1 || exit
     [ ! -d /run/xen ] && mkdir -p /run/xen
     modprobe -q xen-evtchn xen-gnttalloc || exit 1
     mountpoint -q /proc/xen || mount -t xenfs xenfs /proc/xen