cgroups: Redefine what "unlimited" means wrt memory limits

cgroups: Redefine what "unlimited" means wrt memory limits

Since kernel 3.12 (commit 34ff8dc08956098563989d8599840b130be81252 in
linux-stable.git in particular) the value for 'unlimited' in cgroup
memory limits changed from LLONG_MAX to ULLONG_MAX.  Due to rather
unfortunate choice of our VIR_DOMAIN_MEMORY_PARAM_UNLIMITED constant
(which we transfer as an unsigned long long in Kibibytes), we ended up
with the situation described below (applies to x86_64):

 - 2^64-1 (ULLONG_MAX) -- "unlimited" in kernel = 3.12

 - 2^63-1 (LLONG_MAX) -- "unlimited" in kernel < 3.12
 - 2^63-1024 -- our PARAM_UNLIMITED scaled to Bytes

 - 2^53-1 -- our PARAM_UNLIMITED unscaled (in Kibibytes)

This means that when any number within (2^63-1, 2^64-1] is read from
memory cgroup, we are transferring that number instead of "unlimited".
Unfortunately, changing VIR_DOMAIN_MEMORY_PARAM_UNLIMITED would break
ABI compatibility and thus we have to resort to a different solution.

With this patch every value greater than PARAM_UNLIMITED means
"unlimited".  Even though this may seem misleading, we are already in
such unclear situation when running 3.12 kernel with memory limits set
to 2^63.

One example showing most of the problems at once (with kernel 3.12.2):
 # virsh memtune asdf --hard-limit 9007199254740991 --swap-hard-limit -1
 # echo 12345678901234567890 >\
/sys/fs/cgroup/memory/machine/asdf.libvirt-qemu/memory.soft_limit_in_bytes
 # virsh memtune asdf
 hard_limit     : 18014398509481983
 soft_limit     : 12056327051986884
 swap_hard_limit: 18014398509481983

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>