I use bridges for KVM F17, but no - virtmanager isn't much help. It doesn't have any nice features for bridges.
Also I don't believe that /etc/sysconfig/network-scripts is effective unless you copy or link the results into /etc/sysconfig/networking/devices
sudo yum -y install bridge-utils
Use 'network.service' and not 'NetworkManager.service' for network configs.
My LAN is 192.168.42.*
I create a bridge called 'br0'
kvm host if 'hypoxylon'
domain is '.localdomain'
Here are the files under /etc/sysconfig/networking/devices
-rw-r--r--. 3 root root 97 Dec 7 02:25 ifcfg-br0
-rw-r--r--. 3 root root 375 Dec 7 02:25 ifcfg-p20p1
[stevea@hypoxylon devices]$ cat ifcfg-br0
This is the local interface on hypoxylon
[stevea@hypoxylon devices]$ cat ifcfg-p20p1
# new for bridge
I think I could clean the above up, but ... no time.
Some of the non-obvious junk like PEERDNS seems to be necessary
Then in virt-manager when you create a virtual-machine, select the VM's network as
"Specify Virtual device name", and select "br0" (your bridge name) as the "Bridge name".
Then the VMs show up on the local net, can DHCP there, just like any real system.
In the past the bridge was dramatically faster performance than the forwarding method, but that's no longer true. There is no great speed difference, but of course it's much more convenient to have the VMs participate on the LAN.
This may help