Add support for 'passthru' mode when creating a macvlan/macvtap device
which allows takeover of the underlying device and passing it to a KVM
guest using virtio with macvtap backend.

Only one macvlan device is allowed in passthru mode and it inherits
the mac address from the underlying device and sets it in promiscuous
mode to receive and forward all the packets.
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>