qemu: don't setup cpuset.mems if memory mode in numatune is not 'strict'

qemu: don't setup cpuset.mems if memory mode in numatune is not 'strict'

If the memory mode in numatune is specified as 'preferred' with one node
(such as nodeset='0'), domain's memory is not all in node 0 absolutely.
Assumption that node 0 doesn't have enough memory, memory can be allocated
on node 1 when qemu process startup. Then if we set cpuset.mems to '0',
it may invoke OOM.

Commit 1a7be8c600905aa07ac2d78293336ba8523ad48e changed the former logic of
checking memory mode in virDomainNumatuneGetNodeset. This patch adds the
check as before.

Signed-off-by: Wang Rui <moon.wangrui@huawei.com>