You need to create an interface that's connected to the vlan (like "eth1.2102" for vlan 2102). Then you create a bridge ("br2102") that has the vlan interface as a member. You can use the /etc/sysconfig/network-scripts entries to do that, don't assign IPs to those interfaces. Then use the bridge interface in the qemu/kvm configration as the interface for the virtual machine.
Config example (Tested on Centos 6.0):
HWADDR=00:15:17:dc:ff:ff # use your mac here
"ifup br2102" should bring up all those interfaces. Be sure to have this in /etc/sysctl.conf:
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
Troubleshooting: use "tcpdump -i eth1.2102 -n" and tcpdump -i br2102 -n" to see the traffic on those interfaces. "brctl show" is handy to see the bridge config with the virtual machine interface on the bridge added after startup. PXE booting has a problem, you need to interrupt at the PXE prompt for around 10 seconds before the interface is available. Otherwise the PXE rom doesn't get an IP.