Как добавить пустой br1 интерфейс чтоб KVM к нему подключился?

Для Вас даже документацию написали

# Bridging (802.1d)
# For bridging support emerge net-misc/bridge-utils

# To add ports to bridge br0
#bridge_br0="eth0 eth1"
# or dynamically add them when the interface comes up
#bridge_add_eth0="br0"
#bridge_add_eth1="br0"

# NOTE: If you want to manually start an empty bridge, and then dynamically add
# ports to it you must set at least one of the following variables based on the
# interface name, so that we can pick it up from your configuration. Even an
# empty value variable is fine, but at least one of them must be set:
# brctl_IFVAR

# You need to configure the ports to null values so dhcp does not get started
#config_eth0="null"
#config_eth1="null"