qemuSetupMemoryCgroup: Handle hard_limit properly
qemuSetupMemoryCgroup: Handle hard_limit properly

Since 16bcb3 we have a regression. The hard_limit is set
unconditionally. By default the limit is zero. Hence, if user hasn't
configured any, we set the zero in cgroup subsystem making the kernel
kill the corresponding qemu process immediately. The proper fix is to
set hard_limit iff user has configured any.

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