lxc: fuse: Fill in MemAvailable for /proc/meminfo

'free' on Fedora 23 will use MemAvailable to calculate its 'available'
field, but we are passing through the host's value. Set it to match
MemFree, which is what 'free' will do for older linux that don't have
MemAvailable

https://bugzilla.redhat.com/show_bug.cgi?id=1300781
(cherry picked from commit c7be484d)

src/lxc/lxc_fuse.c

@@ -174,6 +174,14 @@ static int lxcProcReadMeminfo(char *hostpath, virDomainDefPtr def,
                     virDomainDefGetMemoryActual(def))) {
             virBufferAsprintf(new_meminfo, "MemFree:        %8llu kB\n",
                               (meminfo.memtotal - meminfo.memusage));
+        } else if (STREQ(line, "MemAvailable") &&
+                   (virMemoryLimitIsSet(def->mem.hard_limit) ||
+                    virDomainDefGetMemoryActual(def))) {
+            /* MemAvailable is actually MemFree + SRReclaimable +
+               some other bits, but MemFree is the closest approximation
+               we have */
+            virBufferAsprintf(new_meminfo, "MemAvailable:   %8llu kB\n",
+                              (meminfo.memtotal - meminfo.memusage));
         } else if (STREQ(line, "Buffers")) {
             virBufferAsprintf(new_meminfo, "Buffers:        %8d kB\n", 0);
         } else if (STREQ(line, "Cached")) {