tools/xenstore: add support for delaying execution of a xenstore request

tools/xenstore: add support for delaying execution of a xenstore request

Today a Xenstore request is processed as soon as it is seen by
xenstored. Add the framework for being able to delay processing of a
request if the right conditions aren't met.

Any delayed requests are executed at the end of the main processing
loop in xenstored. They can either delay themselves again or just do
their job. In order to enable the possibility of a timeout, the main
loop will be paused for max one second if any requests are delayed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>