Fix incorrect XPath for guestrecipe in beaker-watchdog

[kndk]

When an external watchdog expires for a guest recipe in Virtualization workflow, the following error occurs:

beaker-watchdog[2548]: bkr.labcontroller.watchdog INFO External Watchdog Expired for recipe 226016 on system virt-guest.localdomain
beaker-watchdog[2548]: bkr.labcontroller.proxy DEBUG get_recipe recipe_id:226016
beaker-server[10091]: bkr.server.xmlrpccontroller DEBUG Handling recipes.to_xml (226016,)
beaker-server[10091]: bkr.server.xmlrpccontroller DEBUG Time: 0:00:00.029563 recipes.to_xml (226016,)
beaker-watchdog[2548]: bkr.labcontroller.watchdog ERROR Failed to abort expired watchdog
beaker-watchdog[2548]:  Traceback (most recent call last):
beaker-watchdog[2548]:    File "/usr/lib/python2.7/site-packages/bkr/labcontroller/watchdog.py", line 141, in poll
beaker-watchdog[2548]:      self.abort(recipe_id, system)
beaker-watchdog[2548]:    File "/usr/lib/python2.7/site-packages/bkr/labcontroller/watchdog.py", line 100, in abort
beaker-watchdog[2548]:      task_id = task.get('id')
beaker-watchdog[2548]:  UnboundLocalError: local variable 'task' referenced before assignment
beaker-watchdog[2548]: bkr.labcontroller.watchdog DEBUG Polling for active watchdogs
beaker-server[25605]: bkr.server.xmlrpccontroller DEBUG Handling recipes.tasks.watchdogs ('active',)
beaker-server[25605]: bkr.server.xmlrpccontroller DEBUG Time: 0:00:00.006841 recipes.tasks.watchdogs 

Watchdog.abort() method looks for a guest recipe using XPath 'recipeSet/guestrecipe', but fails because the actual XPath is 'recipeSet/recipe/guestrecipe'.

It then falls back to the parent recipe at 'recipeSet/recipe'. If that recipe has already finished, no running task is found and task remains undefined, resulting in an UnboundLocalError.