qemu: Set up EMULATOR thread and cpuset.mems before exec()-ing qemu
    
    It's funny how this went unnoticed for such a long time. Long
    story short, if a domain is configured with
    VIR_DOMAIN_NUMATUNE_MEM_STRICT libvirt doesn't really honour
    that. This is because of 7e72ac787848 after which libvirt allowed
    qemu to allocate memory just anywhere and only after that it used
    some magic involving cpuset.memory_migrate and cpuset.mems to
    move the memory to desired NUMA nodes. This was done in order to
    work around some KVM bug where KVM would fail if there wasn't a
    DMA zone available on the NUMA node. Well, while the work around
    might stopped libvirt tickling the KVM bug it also caused a bug
    on libvirt side: if there is not enough memory on configured NUMA
    node(s) then any attempt to start a domain must fail. Because of
    the way we play with guest memory domains can start just happily.
    
    The solution is to move the child we've just forked into emulator
    cgroup, set up cpuset.mems and exec() qemu only after that.
    
    This basically reverts 7e72ac787848b7434c9 which was a workaround
    for kernel bug. This bug was apparently fixed because I've tested
    this successfully with recent kernel.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Martin Kletzander <mkletzan@redhat.com>