webadmin: Fixed network sorting in VM networks popup

Used LexoNumericComparator to sort the networks by name, with regard to
possible null entries (null last).

Change-Id: I5bf460633674135ba93babb6b0ffa851ca79d4d6
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=911027
Signed-off-by: Lior Vernia <lvernia@redhat.com>
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>