xencommons: redirect serial and parallel to /dev/null

xencommons: redirect serial and parallel to /dev/null

Upstream QEMU doesn't support -nographic with -daemonize unless monitor,
serial and parallel outputs are all redirected:

/* According to documentation and historically, -nographic redirects
 * serial port, parallel port and monitor to stdio, which does not work
 * with -daemonize.  We can redirect these to null instead, but since
 * -nographic is legacy, let's just error out.
 * We disallow -nographic only if all other ports are not redirected
 * explicitly, to not break existing legacy setups which uses
 * -nographic _and_ redirects all ports explicitly - this is valid
 * usage, -nographic is just a no-op in this case.
 */

Considering that we do want to redirect them to /dev/null anyway, do so.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- handled reject vs 26352:9a1610c1e564 "xencommons: Stop QEMU in
         do_stop()" and rewrapped the resulting long line ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>