recent libnfs versions support logging debug messages. Add
support for it in qemu through an URL parameter.

Example:
 qemu -cdrom nfs://127.0.0.1/iso/my.iso?debug=2Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NFam Zheng <famz@redhat.com>
Message-id: 1447052973-14513-1-git-send-email-pl@kamp.de
Signed-off-by: NJeff Cody <jcody@redhat.com>