CA-333687: Fix boot hang caused by CPU hotplug rule
CA-333687: Fix boot hang caused by CPU hotplug rule

The guest agent installs a udev rule which triggers when a CPU is added.
The rule checks whether it is running under Xen using xenstore-exists,
and then onlines the vCPU.

This is problematic during boot because xenstore-exists tries to connect
to xenstore by accessing /proc/xen/xenbus which is on a separate mount
that either doesn't exist at that point or isn't usable. The result is a
hang and systemd-udev-settle.service stalling boot.

Fix it by simply checking for the presence of /dev/xen/xenbus to
determine whether the agent is running under Xen.

Signed-off-by: Ross Lagerwall <d769abb2ab51a4b3c6d56e5ec4ccdd860b82f07a@citrix.com>

xenserver/xe-guest-utilities,xenserver/xe-guest-utilities,xenserver/xe-guest-utilities