[CA-119964] Remove a null dereferece which causes an exception

Also:
Free a XenStore allocation using XSFree
Spell XenServer correctly
Remove the debug eventlog message on shutdown (messages on resume from
  suspend, wmi failure and unexpected error conditions remain as these
  will be useful for issue identification)

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>