They were using the xenbus_dev_fatal() function which would
change the state of the connection immediately. Which is not
what we want when we advertise optional features.

So make 'feature-discard','feature-barrier','feature-flush-cache'
optional.
Suggested-by: NJan Beulich <JBeulich@suse.com>
[v1: Made the discard function void and static]
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>