CA-121385 Allow creation of VLAN 0 in XenCenter.

This applies to the New Network Wizard and the Network Properties.

- The VLAN can be set to 0 only for Creedence or greater hosts on the vSwitch backend.
- When VLAN 0 is selected, an info is shown next to the control, saying "VLAN 0 will receive all traffic not on any other VLAN"
- Also fixed an error on New Network Wizard, where the "VLAN in use" message was still visible when changing the NIC.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>