qemu: Set swap_hard_limit before hard_limit

qemu: Set swap_hard_limit before hard_limit

Setting hard_limit larger than previous swap_hard_limit must fail,
it's not that good if one wants to change the swap_hard_limit
and hard_limit together. E.g.

% virsh memtune rhel6
hard_limit     : 1000000
soft_limit     : 1000000
swap_hard_limit: 1000000

% virsh memtune rhel6 --hard-limit 1000020 --soft-limit 1000020 \
--swap-hard-limit 1000020 --live

This patch reorder the limits setting to set the swap_hard_limit
first, hard_limit then, and soft_limit last if it's greater than
current swap_hard_limit. And soft_limit first, hard_limit then,
swap_hard_limit last, if not.