securable: Do not try to secure inner classes

Previously the class decorator was not careful enough, "securing" any
callable, which matches inner classes.

This patch corrects the type check so only unbound methods are matched
and add more tests, ensuring this will not break in future version.

Change-Id: Ic5ea86b94ca5f07753e2376de8e9e7228a2a147e
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24487
Reviewed-by: Dan Kenigsberg <danken@redhat.com>