Generate libvirt's interface XML definition for vlan tagged bridge

To support vlan tagged virtual network, kimchi needs create the underlying
vlan and bridge interface. Libvirt's interface driver can do it with
a given XML definition. This patch targets to generate the XML according
to the interface and vlan id.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Support creating vlan tagged virtual network

It creates a vlan interface on top of the given nic or bond interface,
and bridge the vlan interface to a new created bridge, which is used to
forward VM's traffic. So all packets transmitted from VM will be tagged
before it departs from the physical network interface.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
UI: All user to specify a vlan ID for a virtual network.

It adds a checkbox to provide choice of enabling vlan. If it's checked,
it all user to type the vlan ID.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>