fw_cfg: Adopt /opt/RFQDN convention

fw_cfg: Adopt /opt/RFQDN convention

FW CFG's primary user is QEMU, which uses it to expose configuration
information (in the widest sense) to Firmware.  Thus the name FW CFG.

FW CFG can also be used by others for their own purposes.  QEMU is
merely acting as transport then.  Names starting with opt/ are
reserved for such uses.  There is no provision, however, to guide safe
sharing among different such users.

Fix that, loosely following QMP precedence: names should start with
opt/RFQDN/, where RFQDN is a reverse fully qualified domain name you
control.

Based on a more ambitious patch from Michael Tsirkin.

Cc: Gerd Hoffmann <[email protected]>
Cc: Gabriel L. Somlo <[email protected]>
Cc: Laszlo Ersek <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Acked-by: Gabriel Somlo <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>