Make _filterDevices() return a generator consistently (#1122081)

Related: fed#1122081

Previously, it returned a list when incomplete was True and a
generator when it was False.

Signed-off-by: mulhern <amulhern@redhat.com>
Make prefering leaves the default in getDeviceByPath (#1122081)

Resolves: fed#1122081

Preferring leaves was the default until commit
822d92457e36768b9daa6a3ec689e494e9ba07df which deliberately but
mistakenly reversed the preference.

Signed-off-by: mulhern <amulhern@redhat.com>