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: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>