machine: Polish -machine xxx,help

The "-machine xxx,help" prints kernel-irqchip possible values as
"OnOffSplit", this adds separators to the printed line.

Also, since only lower case letters are specified in qapi/common.json,
this changes the letter cases too.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>