libxl: increase hotplug timeout to 40s

libxl: increase hotplug timeout to 40s

The default libxl timeout for hotplug scripts execution is too low, when
launching 40 HVM guests in parallel, all using the same file as disk,
execution times of ~20s are expected. Increase the timeout to 40s in order
to be sure hotplug scripts have enough time to execute.

This is a short term solution.

Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>