qga: add --dump-conf option

qga: add --dump-conf option

This new option allows to review the agent configuration,
and ease the task of writing a configuration file.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
* removed unecessary keyfile != NULL prior to free
* documented --dump-conf is qemu-ga --help output
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>