xl: track size of diskws with a dedicated counter

xl: track size of diskws with a dedicated counter

The num_disks field can change during guest lifetime. Don't use that as
the size of diskws, use a dedicated counter instead.

Also free diskws and reset diskws to NULL after disabling events so that
it will be automatically re-created when the guest reboots.

Reported-by: Fatih Acar <fatih.acar@gandi.net>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Fatih Acar <fatih.acar@gandi.net>