9p caching with cache=loose and cache=fscache

[Luis Chamberlain]

We've embraced 9p on kdevops [0] for support to let a guest use a host's
path to install Linux. This works well except I notice that if the host
makes new changes the guest does not see it. This just means the guest
has to reboot every time before running 'make modules_install install -j 8'
and it is unexpected and not user friendly.

I wanted to see if someone was working on fixing this yet. I've tested
with both cache=loose and cache=fscache. We use msize 131072, no
security and virtio-9p-pci.

[0] https://github.com/linux-kdevops/kdevops

  Luis