Don't crash in _checkImageState if there's no image.os_plugin

We've seen some tasks lately where Koji crashed trying to do
`image.os_plugin.abort()`, e.g.:

https://koji.fedoraproject.org/koji/taskinfo?taskID=89750587
https://koji.fedoraproject.org/koji/taskinfo?taskID=89671944

The fix seems fairly simple: only try and abort if os_plugin
actually exists.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

builder/kojid

@@ -4164,7 +4164,8 @@ class BaseImageTask(OzImageTask):
             if scrnshot:
                 ext = scrnshot[-3:]
                 self.uploadFile(scrnshot, remoteName='screenshot.%s' % ext)
-            image.os_plugin.abort()  # forcibly tear down the VM
+            if image.os_plugin:
+                image.os_plugin.abort()  # forcibly tear down the VM
             # TODO abort when a task is CANCELLED
             if not self.session.checkUpload('', os.path.basename(self.ozlog)):
                 self.tlog.removeHandler(self.fhandler)