Handle exceptions on reading package lists

In some cases it might be possible that the package database
is not accessible to non-privileged users.
This patch handles and logs this issue and prevents the exception
from stopping the guest agent thread with an unhandled exception.

Change-Id: Id44210235a3bd8fadf91cb919896c1abe8b4c250
Bug-Url: https://bugzilla.redhat.com/1117632
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>