Commit e3435caf fixed hot-plugging of vcpus with strict memory pinning
on NUMA hosts, but unfortunately it also broke updating number of vcpus
for offline guests using our API.

The issue is that we try to create a cpu cgroup for non-running guest
which fails as there are no cgroups for that domain. We should create
cgroups and update cpuset.mems only if we are hot-plugging.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>