libxl: convert libxl_device_disk_add to an async op

libxl: convert libxl_device_disk_add to an async op

This patch converts libxl_device_disk_add to an ao operation that
waits for device backend to reach state XenbusStateInitWait and then
marks the operation as completed. This is not really useful now, but
will be used by later patches that will launch hotplug scripts after
we reached the desired xenbus state.

As usual, libxl_device_disk_add callers have been modified, and the
internal function libxl__device_disk_add has been used if the call was
inside an already running ao.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- drop hunk modifying libxl_cdrom_insert which is not needed after
         25670:3666e9712eaf "libxl: make libxl_cdrom_insert async" ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>