qapi: Add human mode to StringOutputVisitor This will be used by "info qtree". For numbers it prints both the decimal and hex values. For sizes it rounds to the nearest power of 2^10. For strings, it puts quotes around the string and separates NULL and empty string. Reviewed-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Andreas Färber <afaerber@suse.de>