vmstatus: fix memory usage value including cache sizes

vmstatus: fix memory usage value including cache sizes

The cgroup value memory.usage_in_bytes includes the memory used by
file buffers and other caches, resolve this by getting the cache
value from the memory.stat file and substract it from
memory.usage_in_bytes when calculating the current memory usage of
the CT.
This results in the same value as a `free` call from the container
does (when not including the buffered data), at least with a free
version which uses data from /proc and not the sysinfo() syscall.

Addresses partly the bug #1139

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
CC: Wolfgang Bumiller <w.bumiller@proxmox.com>