tools/xenstored: Limit the number of requests a connection can delay

tools/xenstored: Limit the number of requests a connection can delay

Currently, only liveupdate request can be delayed. The request can only
be performed by a privileged connection (e.g. dom0). So it is fine to
have no limits.

In a follow-up patch we will want to delay request for unprivileged
connection as well. So it is best to apply a limit.

For now and for simplicity, only a single request can be delayed
for a given unprivileged connection.

Take the opportunity to tweak the prototype and provide a way to
bypass the quota check. This would be useful when the function
is called from the restore code.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Juergen Gross <jgross@suse.com>