numa_conf: Make virDomainNumaSetNodeCpumask() return void

numa_conf: Make virDomainNumaSetNodeCpumask() return void

There is only one caller of virDomainNumaSetNodeCpumask() which
checks for the return value but because the function will return
NULL iff the @cpumask was NULL in the first place. But in that
place @cpumask can't be NULL because it was just allocated by
virBitmapParse().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>