doc: fix hardcoded helper path

doc: fix hardcoded helper path

The install directory of qemu-bridge-helper is configurable,
but we use a fixed path in the documentation.

DEFAULT_BRIDGE_HELPER macro isn't available in texi mode,
we should always use "/path/to/" prefix for dynamic paths
(e.g.: /path/to/image, /path/to/linux, etc).

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>