qemu: virtiofs: cache: use 'never' instead of 'none'

The new option style renamed one of the cache modes.

https://issues.redhat.com/browse/RHEL-50329



Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>