How to share a folder in libvirt+qemu/kvm with Linux guest and Linux host?

I am running a VM on a Linux host using qemu+kvm and libvirt, the guest system is CentOS 7. I wish to share a folder between the two such that host:/some/dir becomes accessible as guest:/some/other/dir. I wish to minimise effort and cannot re-compile the kernel on either side. I experimented with plan9 but cannot get it to run (the p9 dkms module won't compile on the CentOS 7 guest). There must be a better way? Performance is secondary as it does not work at all right now.