VLAN field in networks table should be constrained

[Salvatore Orlando]

nova-manage currently allows to create networks with an arbitrary integer for the VLAN ID.
For instance, the following comands result in a record successfully added to the networks table:

nova-manage network create 10.0.4.0/24 1 256 -25
nova-manage network create 10.0.4.0/24 1 256 5500

However, vconfig will fail when trying to create the vlan interface with the following error:
ERROR: trying to add VLAN #XXXX to IF -:eth0:- error: Numerical result out of range

the create_networks method of the VlanManager class should probably return an error code if vlan_start + num_networks > 4094