add vnet

add vnet

This allow to define bridges with a tag (can be vxlan or vlan tag),
and use configuration from transportzone

/etc/pve/network/vnet.cfg

vnet1:
      tag 2
      transportzone vlanzone1
      name network1
      ipv4 10.0.0.1
      ipv6 2a03:2880:f003:c07:face:b00c::2
      mtu 1500

vnet2:
      transportzone vlanzone1
      tag 3
      name network2
      ipv6 2a03:2880:f003:c07:face:b00c::2

vnet3:
      transportzone vxlanmulticastzone1
      tag 100000
      name network3
      mtu 1400

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>