conf: Factor out vcpus parsing part from virDomainCachetuneDefParse
Extract vcpus parsing part from virDomainCachetuneDefParse into one
function called virDomainResctrlParseVcpus. So that vcpus parsing logic
can be reused by other resource control technologies. Adjust error
message and use node->name so that the error message can fit to all
technologies.
Signed-off-by: Bing Niu <bing.niu@intel.com> Reviewed-by: John Ferlan <jferlan@redhat.com>