> +    { "machine", "vmport", QEMU_CAPS_MACHINE_VMPORT_OPT },

Ouch.  qemu commit 0a7cf21 fixes what would have been a regression in
2.3 at exposing "mem-merge" through query-command-line-options, but it
does NOT expose "vmport", which is a per-architecture option rather than
a generic -machine option.  Which means that even though qemu 2.2
(perhaps wrongly) advertised "vmport" for all machines (even when it was
not supported), 2.3 will not advertise it, and we are hoping for a
better solution in 2.4 for properly advertising vmport on an
as-appropriate basis.