title: xen: domu require more memory than mem-set size
body: <p>Assume that we have install two guest os in xen, named dom1 and dom2. Now, I start the dom1 and dom2, and both are configured with max-mem=4096M; note that my physical memory is 16GB.
  While dom1 and dom2 have run for a long time, I set the memory size of dom1 and dom2 with mem-set:1024M. So my question 1.what happened to the freed memory(4G-1G=3G) of dom1 and dom2? I have check the memory size of dom1 and dom2, about 1G. So the 3G memory of dom1 and dom2 have returned to dom0, or returned to hypervisor? 2. If dom1 run a application, and it require memory size more than 1G, so what happened to the memory of dom1? I have also checked the memory size by using "xm list". the memory of dom1 is still 1024M. so why hypervisor dynamic allocate the dom1 more memory with the method of balloon driver?</p>