qemu-options.hx: improve -m description Add memory hotplug options to the command-line format. Also, add a complete command-line example and improve description. Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com> Reviewed-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Paulo Vital <paulo.vital@profitbricks.com>