Description of problem:
Every time I get the update information through yumex-dnf or dnfdragora, dnfdaemon gives a traceback (see below).

Version-Release number of selected component (if applicable):
dnfdaemon-0.3.17-1.fc26.noarch

How reproducible:
always

Steps to Reproduce:
1. open yumex-dnf or dnfdragora
2. display update information for any package

Actual results:
Crash

Expected results:
no crash

Additional info:
This causes a crasher in dnfdragora, see bug #1444825.

Traceback:
g-io-error-quark: GDBus.Error:org.freedesktop.DBus.Python.AttributeError: Traceback (most recent call last):
  File "/usr/lib64/python3.6/site-packages/dbus/service.py", line 707, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/usr/lib/python3.6/site-packages/dnfdaemon/server/__init__.py", line 83, in newFunc
    rc = func(*args, **kwargs)
  File "/usr/share/dnfdaemon/dnfdaemon-system", line 273, in GetAttribute
    value = self.get_attribute(pkg_id, attr)
  File "/usr/lib/python3.6/site-packages/dnfdaemon/server/__init__.py", line 343, in get_attribute
    value = json.dumps(self._get_fake_attributes(po, attr))
  File "/usr/lib/python3.6/site-packages/dnfdaemon/server/__init__.py", line 909, in _get_fake_attributes
    return self._get_update_info(po)
  File "/usr/lib/python3.6/site-packages/dnfdaemon/server/__init__.py", line 846, in _get_update_info
    value = updinfo.advisories_list()
  File "/usr/lib/python3.6/site-packages/dnfdaemon/server/backend.py", line 524, in advisories_list
    e[field] = getattr(adv, field)
AttributeError: '_hawkey.Advisory' object has no attribute 'filenames'
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