Issue #365: Preventing duplicate Bridged VLAN tagged network.

While creating a Bridged VLAN tagged network using the same
configuration of a previous network (destination and VLAN ID) but
with different names, Kimchi allows the creation process.

This patch prevents the creation of the second Bridged VLAN tagged
network, checking if already has some network with the same bridge
configured.

https://github.com/kimchi-project/kimchi/issues/365

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>