libxl: fix removal of secondary consoles

libxl: fix removal of secondary consoles

Secondary consoles are processed by libxl with the rest of the
devices by calling libxl__initiate_device_remove that waits for the
device to reach state 6 before procceeding with the removal.

When libxl is destroying the console devices, Qemu is already dead or
dying, and xenconsoled completely ignores the state backend entry for
console devices, since it performs the cleanup based on the result of
reads/writes to the tty.

Since we don't want to execute hotplug scripts for consoles, leave the
behaviour as it was previously, and just nuke the backend/frontend
xenstore entries by calling libxl__device_destroy.

Report: http://markmail.org/message/yqgppcsdip6tnmh6

Reported-by: Ian Campbell <ian.campbell@eu.citrix.com>
Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>