libxl: do not fail device removal if backend domain is gone

libxl: do not fail device removal if backend domain is gone

Backend domain may be independently destroyed - there is no
synchronization of libxl structures (including /libxl tree) elsewhere.
Backend might also remove the device info from its backend xenstore
subtree on its own.

We have various cases (not comprehensive list):

 - both frontend and backend operational: after setting
   be/state=XenbusStateClosing backend wait for frontend confirmation
   and respond with be/state=XenbusStateClosed; then libxl in dom0
   remove frontend entries and libxl in backend domain (which may be the
   same) remove backend entries
 - unresponsive backend/frontend: after a timeout, force=1 is used to remove
   frontend entries, instead of just setting
   be/state=XenbusStateClosing; then wait for be/state=XenbusStateClosed.
   If that timeout too, remove both frontend and backend entries
 - backend gone, with this patch: no place for setting/waiting on
   be/state - go directly to removing frontend entries, without waiting
   for be/state=XenbusStateClosed (this is the difference vs force=1)

Without this patch the end result is similar, both frontend and backend
entries are removed, but in case of backend gone:
 - libxl waits for be/state=XenbusStateClosed (and obviously timeout)
 - return value from the function signal an error, which for example
   confuse libvirt - it thinks the device remove failed, so is still
   there

If such situation is detected, do not fail the removal, but finish the
cleanup of the frontend side and return 0.

This is just workaround, the real fix should watch when the device
backend is removed (including backend domain destruction) and remove
frontend at that time. And report such event to higher layer code, so
for example libvirt could synchronize its state.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>