If you have solved GPU/audio passthrough, to get OpenElec working you may also need to add the virtio drivers. They are already part of the linux kernel, so with "make menuconfig" I 've produced a config file including the Virtio drivers (net pci, balloon..).
To have a OpenElec+virtio Distro, proceed as follow :
Prepare to compile your own OpenElec distro as described in OpenElec Wiki (http://wiki.openelec.tv/index.php/Compile_from_source) .
Check that you can build the last stable distro (5.0. :
git checkout 5.0.8
PROJECT=Generic ARCH=x86_64 make release
Once you are sure everything is OK, you can put the attached file in OpenElec.tv/projects/Generic/linux/ to replace the original conf file, and compile again, this time with the virtio drivers :
PROJECT=Generic ARCH=x86_64 scripts/clean linux
PROJECT=Generic ARCH=x86_64 make release
linux.x86_64.conf