CA-83837: VBD_unplug is now much more robust

VBD_unplug may be called when the domain has been destroyed (in suspend
by libxenguest) and when the xenstore device has already been shutdown.

All the cleanup steps are now Opt.iters over option types, to do as
much cleaning as possible. Note this depends on the new 'domid_of_uuid'
function which uses xenstore rather than the domain info list.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>