spec: Fix python3-libvirt requirement in client-qemu package
The %{release} part of the requirement is just wrong as python bindings
are not rebuilt anytime libvirt release is increased, which means the
client-qemu package may require nonexistent release of python bindings.
The %{version} part is not wrong, but it's too strict for no reason as
the virt-qemu-qmp-proxy script will work happily even with ancient
python bindings. And since all distros supported by libvirt.spec already
contain python3-libvirt, we can depend on the first package called this
way.