Libvirt has supported qemu commandline option passthrough for qemu/kvm VMs for quite a while. The format for it is a bit of a pain though since it requires setting a magic xmlns value at the top of the domain XML. Basically doing it by hand kinda sucks.
In the recently released virt-manager 1.4.1, we added a virt-install/virt-xml option --qemu-commandline that tweaks option passthrough for new or existing VMs. So for example, if you wanted to add the qemu option string '-device FOO' to an existing VM named f25, you can do:
./virt-xml f25 --edit --confirm --qemu-commandline="-device FOO"
The output will look like: