check out https://wiki.qemu.org/Documentation/9psetup

I've used this. It works. I like the fact that a normal, non-root user can write to the shared directory from the guest system. I seem to recall that on VirtualBox I always had to be root in order to do that.

sshfs user@host-ip:/home/user/some-subdirectory  some-mountpoint-in-the-VM

fusermount -u some-mountpoint-in-the-VM   # to un-mount the share

I've done it with nfs, too. That's a little more involved. You can even set it up with autofs so the share mounts automatically when you access the mountpoint.