opts: produce valid command line in qemu_opts_print

opts: produce valid command line in qemu_opts_print

This will let us print options in a format that the user would actually
write it on the command line (foo=bar,baz=asd,etc=def), without
prepending a spurious comma at the beginning of the list, or quoting
values unnecessarily.  This patch provides the following changes:
* write and id=, if the option has an id
* do not print separator before the first element
* do not quote string arguments
* properly escape commas (,) for QEMU

Signed-off-by: Kővágó, Zoltán <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>