I tested spice vdagent, spice audio and spice usbredirection with qemu 
upstream and xen-unstable manually, is all working.
I'm start to write some patches to have all spice features on xen 4.3.

About vdagent probably no problem.

About spice audio on test I actually set this variable manually:

export QEMU_AUDIO_DRV=spice

I need know how to setup this env variable but for a given hvm domU start.
In other word I need to set this env variable on a per domU basis (if 
spiceaudio if setted in cfg).

About usb redirection the qemu parameters to add are similar to this:

device_model_args=["-readconfig","/etc/xen/ich9-ehci-uhci.cfg","-chardev","spicevmc,name=usbredir,id=usbredirchardev1","-device","usb-redir,chardev=usbredirchardev1,id=usbredirdev1,bus=ehci.0,debug=3","-chardev","spicevmc,name=usbredir,id=usbredirchardev2","-device","usb-redir,chardev=usbredirchardev2,id=usbredirdev2,bus=ehci.0,debug=3","-chardev","spicevmc,name=usbredir,id=usbredirchardev3","-device","usb-redir,chardev=usbredirchardev3,id=usbredirdev3,bus=ehci.0,debug=3"]

Probably is not good point to external generic file (on my test 
/etc/xen/ich9-ehci-uhci.cfg), someone can tell me the best way for do this?

About xl cfg parameters can be good add spiceaudio=0|1 and 
spiceusbredirection=0|1 both with default 0?