https://bugzilla.redhat.com/show_bug.cgi?id=896092 mentions that
qemu 1.4 and earlier only accept a simple start-stop range for
the cpu=... argument of -numa.  Libvirt would attempt to use
-numa cpu=1,3 for a disjoint range, which did not work as intended.

Upstream qemu will be adding a new syntax for disjoint cpu ranges
in 1.5; but the design for that syntax is still under discussion
at the time of this patch.  So for libvirt 1.0.3, it is safest to
just reject attempts to build an invalid qemu command line; in the
future, we can add a capability bit and translate to the final
accepted design for selecting a disjoint cpu range in numa.

* src/qemu/qemu_command.c (qemuBuildNumaArgStr): Reject disjoint
ranges.