Xen's automated test system is reporting[1] that something between bfb7b58b3068 and 136d4ec190af (for reference the previous pass is[2]) has broken booting winxp under Xen with Seabios.
Logs for one of the failures can be found at[3] (reached by following the email's link to [4] and clicking the header of a failing column). The VNC screenshot of the guest shows "Setup is inspecting your hardware configuration..." (at least for those which have such a screenshot, not sure why some don't).
I've not investigated more thoroughly yes, just posting in case something obvious leaps out at someone. The automated test is currently bisecting the issue, once it is done I'll let you know the result.
[1] http://lists.xen.org/archives/html/xen-devel/2014-10/msg03543.html [2] http://lists.xen.org/archives/html/xen-devel/2014-10/msg01993.html [3] http://www.chiark.greenend.org.uk/~xensrcts/logs/31223/test-amd64-i386-xl-qe... [4] http://www.chiark.greenend.org.uk/~xensrcts/logs/31223/