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Eduardo Habkost explained the value in RHEL6 BZ 1030290 comment 2:

  The QEMU man page is about QEMU as a whole, not about specific machine-types 
  (which may have lower limits), or about specific accelerator limits (KVM has 
  lower limits), or about what Red Hat supports.

  QEMU itself supports up to 255 CPUs, but KVM and the rhel machine-types don't.

RHEL7 QEMU exits gracefully, with a clear stderr message containing the maximal number of supported VCPUs.