xenfront: guest-side implementation of flush

Not all Xen versions implement the barrier feature in their pv disks. Such is
the case in Amazon EC2. For those, we should interpret the flush request and
wait for the requests until they are all handled. Also, we can still try to use
one of either barriers or flush-disk operations before we resort to guest-side
software implementation. Our driver currently only tests for one of them, so
this patch also implements flush requests.

Luckily, the implementation of xb_dump (which we don't currently use) needs to
do that as well, and this is also quite well isolated in xb_quiesce(). All we need
to do is call xb_quiesce() if flushing is not available in our backend