A small AppVM (say, with 100MB total) can go below prefmem, and
still not be assigned memory, because of the MIN_TOTAL_MEMORY_TRANSFER
threshold.
So, if AppVM is below prefmem, allow for smaller mem-sets.
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%.
