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%.
Instead of hardcoded savefile name, use a symlink in
/var/run/qubes. Tools should set this symlink to a correct
savefile. Also, test whether the savefile is older than the
template root.img.
