Simplify reply logic in xenstored. Maintain a linked list

Simplify reply logic in xenstored. Maintain a linked list
of pending replies that are sent out in order.

Currently we only read new requests when the reply list is
empty. In fact there is no good reason for this restriction.
Another interesting point is that (on my test machine)
hotplug blk setup fails if xenstored_client connects to
xenstored via the unix domain socket rather than through the
kernel --- this points to some user/kernel races that are
'fixed' by the extra serialisation of the in-kernel mutexes.
It definitely needs looking into.

Signed-off-by: Keir Fraser <keir@xensource.com>