I cannot reproduce this bug, list my steps here, if I got something wrong, please tell me what to do

Steps:
1. Create br0 and make sure I can connect to internet via it.
2. Create a guest with br0 as its NIC.
3. Create br1 by using a NIC cannot access internet.
4. Create the second guest, but choose br0 as its NIC(on my machine, br1 will be its default option at this step)
5. Start the guest and check the network.

Actual result:
1. Network in second guest works well. And I can see its NIC is br0, not br1

My build:
libvirt-0.10.1-1.el6.x86_64

This may be related or not but I am running into a similar difficulty on a separate 6.3 kvm host, albeit one with only one defined bridge.  Using virt-manager to create a new guest instance, for all instances created after the first, results in an guest that does not have Internet connectivity.  The first guest created has Internet connectivity but no others do. This is after the eth0 configuration is completed and the network is restarted on both guests of course.

In the following example xnet241 was created first and has Internet connectivity, Xnet242 was created later and does not have Internet connectivity. Both were created through virt-manager, although I cannot say with certainty both were created with the same version given the number of updates recently.

Running diff on the /etc/libvirt/qemu/definition files shows this:

diff /etc/libvirt/qemu/xnet241.harte-lyne.ca.xml /etc/libvirt/qemu/xnet242.harte-lyne.ca.xml
4c4
<   virsh edit xnet241.harte-lyne.ca
---
>   virsh edit xnet242.harte-lyne.ca
9,12c9,12
<   <name>xnet241.harte-lyne.ca</name>
<   <uuid>b0c0c6c5-c464-1dc1-d3a3-f23a6d24e0cd</uuid>
<   <memory unit='KiB'>4194304</memory>
<   <currentMemory unit='KiB'>4194304</currentMemory>
---
>   <name>xnet242.harte-lyne.ca</name>
>   <uuid>b8117efe-63cb-1bcb-0fb8-3442ea8fed3e</uuid>
>   <memory unit='KiB'>2097152</memory>
>   <currentMemory unit='KiB'>2097152</currentMemory>
31c31
<       <source dev='/dev/vg_vhost04/lv_vm_xnet241.harte-lyne.ca_00'/>
---
>       <source dev='/dev/vg_vhost04/lv_vm_xnet242.harte-lyne.ca_00'/>
.  .   . <block device stuff>
59,61c47,49
<     <interface type='bridge'>
<       <mac address='52:54:00:6c:35:1a'/>
<       <source bridge='br0'/>
---
>     <interface type='network'>
>       <mac address='52:54:00:75:f6:52'/>
>       <source network='default'/>
73,75c61
<     <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1'>
<       <listen type='address' address='127.0.0.1'/>
<     </graphics>
---
>     <graphics type='vnc' port='-1' autoport='yes'/>

So, what would make virt-manager select network for this instance instead of bridge?  This behaviour is consistent with the options shown as available inside virt-manager for when the GUI creates a new instance virt-manager shows "Host device eth0 as not bridged". 

However, these are the contents of /etc/sysconfig/network-scripts/ifcfg-br0 and ifcfg-eth0:

cat /etc/sysconfig/network-scripts/ifcfg-br0
BOOTPROTO=static""
DEFROUTE="yes"
DEVICE="br0"
DNS1="216.185.71.33"
DNS2="216.185.71.34"
DNS3="209.47.176.33"
DNS4="209.47.176.34"
DOMAIN="harte-lyne.ca hamilton.harte-lyne.ca mississauga.harte-lyne.ca"
GATEWAY="216.185.71.1"
IPADDR="216.185.71.44"
IPV4_FAILURE_FATAL="yes"
IPV6INIT="no"
NAME="System br0 (vhost04)"
NM_CONTROLLED="no"
ONBOOT="yes"
PREFIX="24"
TYPE="Bridge"
USERCTL="no"
[root@vhost04 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
BOOTPROTO="static"
BRIDGE="br0"
DEVICE="eth0"
NAME="System eth0 - bridged on br0 (vhost04)"
NM_CONTROLLED="no"
ONBOOT="yes"
TYPE="Ethernet"
USERCTL="no"
 

Manually editing the configuration files of the guests without connectivity to use the bridge corrects the connectivity problem.

virt-manager --version
0.9.0

I further note on kvm virtual hosts set up earlier that the virt-manager info display for guests shows an NIC 

"Host device vnetxx (Bridge 'brx') <--drop down menu

but on the most recent kvm host I see this instead:

Specify shared device name         <--(drop down menu)
Bridge name: br0                   <--(text box)

And the available shared device names on the later host do not have and choices with (Bridge 'br0') in them.

the virt-manager on both systems show 0.9.0 as their version number but they seem to behave somewhat differently.

Ennn, OK, I can reproduce this bug now, I noticed your 2nd guest doesn't use network bridge as NIC but using Virtual network 'default' : NAT as its NIC. On my machine, if I configured the eth0 to a bridge, then modify the 1st guest, change the NIC from br0 to Virtual network 'default' : NAT, neither it can visit network, it has nothing related with 1st or 2nd guest.

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