tools/oxenstored: Only quit on SIGTERM when a reload is possible

tools/oxenstored: Only quit on SIGTERM when a reload is possible

Currently when oxenstored receives SIGTERM it dumps its state and quits.  It
is possible to then restart it if --restart is given, however that is not
always safe:

* Domains could have active transactions, and after a restart they would
  either reuse transaction IDs of already open transactions, or get an error
  back that the transaction doesn't exist

* There could be pending data to send to a VM still in oxenstored's
  queue which would be lost

* There could be pending input to be processed from a VM in oxenstored's
  queue which would be lost

Prevent shutting down oxenstored via SIGTERM in the above situations.  Also
ignore domains marked as bad because oxenstored would never talk to them
again.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Reviewed-by: Pau Ruiz Safont <pau.safont@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>