Add the PCI version of vhost-user-fs.

Launch QEMU like this:

  qemu -chardev socket,path=/tmp/vhost-fs.sock,id=chr0
       -device vhost-user-fs-pci,tag=myfs,chardev=chr0
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NSebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Message-Id: <20190930105135.27244-4-dgilbert@redhat.com>
Acked-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>