lxc: fix starting a domain with a cpuset but no numatune

lxc: fix starting a domain with a cpuset but no numatune

 # virsh -c lxc:/// start helloworld
 error: Failed to start domain helloworld
 error: internal error: guest failed to start: Invalid value '1-3'
 for 'cpuset.mems': Invalid argument

Free the cpu mask to avoid reusing it as a mem mask
in virCgroupSetCpusetMems
if virDomainNumatuneMaybeFormatNodeset does not set a mask.

Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>