Make meminfo-writer compute used memory, and report to qmemman only if
it has changed significantly enough. As it is written in C, its code is
much faster that qmemman-server; also in the idle case, it saves on xenstore
communication overhead. Allows to send updates up to 10 times per second,
with CPU load on the VM below 0.1%.
Plus:
- dedicated chain for DNAT to nameservers
- prevent intervm networking. Can be conveniently overriden in necessary cases
by inserting ACCEPT clauses (per VM, probably) at the top of FORWARD
