Fix bugs in server creation, in api/server.py:create_server().
More specifically:
 * Make sure to save() the vm object before calling vm.backend_id
 * Delete the vm instance if the RAPI call fails for any reason
 * Use different variable names for the server name at the API level
   and at the Ganeti backend level
 * Compute a random password *before* the CreateInstance() call to Ganeti
   [not currently used though]

Also patch the VirtualMachine model to safeguard against getting the
backend_id of a yet-unsaved instance of VirtualMachine.