qemu: fix set vcpus on host without NUMA
We don't have to modify cpuset.mems on hosts without NUMA. It also
fixes an error message that you get instead of success if you trying
update vcpus of a guest on a host without NUMA.
error: internal error: NUMA isn't available on this host
Signer-off-by: Pavel Hrdina <phrdina@redhat.com>