xen: perform XenDevice clean-up in XenBus watch handler

xen: perform XenDevice clean-up in XenBus watch handler

Cleaning up offline XenDevice objects directly in
xen_device_backend_changed() is dangerous as xen_device_unrealize() will
modify the watch list that is being walked. Even the QLIST_FOREACH_SAFE()
used in notifier_list_notify() is insufficient as *two* notifiers (for
the frontend and backend watches) are removed, thus potentially rendering
the 'next' pointer unsafe.

The solution is to use the XenBus backend_watch handler to do the clean-up
instead, as it is invoked whilst walking a separate watch list.

This patch therefore adds a new 'inactive_devices' list to XenBus, to which
offline devices are added by xen_device_backend_changed(). The XenBus
backend_watch registration is also changed to not only invoke
xen_bus_enumerate() but also a new xen_bus_cleanup() function, which will
walk 'inactive_devices' and perform the necessary actions.
For safety an extra 'online' check is also added to xen_bus_type_enumerate()
to make sure that no attempt is made to create a new XenDevice object for a
backend that is offline.

NOTE: This patch also includes some cosmetic changes:
      - substitute the local variable name 'backend_state'
        in xen_bus_type_enumerate() with 'state', since there
        is no ambiguity with any other state in that context.
      - change xen_device_state_is_active() to
        xen_device_frontend_is_active() (and pass a XenDevice directly)
        since the state tests contained therein only apply to a frontend.
      - use 'state' rather then 'xendev->backend_state' in
        xen_device_backend_changed() to shorten the code.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190913082159.31338-4-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>