qga: fix fd leak with guest-exec i/o channels

Signed-off-by: Yuriy Pudgorodskiy <yur@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Michael Roth <mdroth@linux.vnet.ibm.com>
* squashed in g_io_channel_shutdown() to match cleanup paths for
  input/output
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
