oxenstored: Protect oxenstored from malicious domains.

oxenstored: Protect oxenstored from malicious domains.

add check logic when read from IO ring, and if error happens,
then mark the reading connection as "bad", Unless vm reboot,
oxenstored will not handle message from this connection any more.

xs_ring_stubs.c: add a more strict check on ring reading
connection.ml, domain.ml: add getter and setter for bad flag
process.ml: if exception raised when reading from domain's ring,
            mark this domain as "bad"
xenstored.ml: if a domain is marked as "bad", do not handle it.

Signed-off-by: John Liu <john.liuqiming@huawei.com>
Acked-by: David Scott <dave.scott@eu.citrix.com>