Have you confirmed that the DHCP offer frame is sent to the broadcast MAC address?
Does the guest system see other frames that are sent to the broadcast MAC address? (Try sending one from an external machine, and also try one from the host itself).
Maybe the host isnt correctly passing broadcast frames to the bridged interface? It could be a problem in the bridging code.
Also, what type of physical interface is eth0 on the guest bridged to? A wired Ethernet connection or a wireless interface? From what I recall I don't think bridging with wireless interfaces is particularly mature as yet.