qdev_monitor: Simplify error handling in qdev_device_add()

Instead of doing the clean-ups on errors multiple times, introduce
a jump label at the end of the function that can be used by all
error paths that need this cleanup.

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1509617407-21191-2-git-send-email-thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>