qemu-option: add help fallback to print the list of options

qemu-option: add help fallback to print the list of options

QDev options accept 'help' (or '?', but that's problematic with shell
globbing) in the list of parameters, which is handy to list the
available options.

Unfortunately, this isn't built in QemuOpts. qemu_opts_parse_noisily()
seems to be the common path for command line options, so place a
fallback to print help, listing the available options.

This is quite handy, for example with qemu "-spice help".

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Eric Blake <[email protected]>