This removes the virNWFilterHashTableFree, virNWFilterHashTablePut
and virNWFilterHashTableRemove methods, in favour of just calling
the virHash APIs directly.

The virNWFilterHashTablePut method was unreasonably complex because
the virHashUpdateEntry already knows how to create the entry if it
does not currently exist.
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>