KVM - Enable Promiscuous Mode for Macvtap Network Interface

To use the relatively new macvtap bridge network interfaces in KVM, the host network interface must be set to promiscuous mode.

Edit /etc/network/interfaces with nano or something and add the up and down lines.
For example...

iface eth0 inet static
up ifconfig $IFACE promisc
#######
# YOUR INTERFACE CONFIG HERE
#######
down ifconfig $IFACE -promisc

Reboot and check if promiscuous mode is enabled: ifconfig and see if the PROMISC flag is set on the interface.