fish: Add --rw option (does nothing yet).

fish: Add --rw option (does nothing yet).

This adds the guestfish --rw option, intended in future
to be required for writing to disk images.

At the moment this does not change the default and so does
nothing.  This patch is intended for backporting to the
stable branches so that we can start to introduce scripts
which use 'guestfish --rw'.