qemu-storage-daemon: Only display FUSE help when FUSE is built-in

qemu-storage-daemon: Only display FUSE help when FUSE is built-in

When configuring QEMU with --disable-fuse, the qemu-storage-daemon
still reports FUSE command line options in its help:

  $ qemu-storage-daemon -h
  Usage: qemu-storage-daemon [options]
  QEMU storage daemon

    --export [type=]fuse,id=<id>,node-name=<node-name>,mountpoint=<file>
             [,growable=on|off][,writable=on|off]
                           export the specified block node over FUSE

Remove this help message when FUSE is disabled, to avoid:

  $ qemu-storage-daemon --export fuse
  qemu-storage-daemon: --export fuse: Invalid parameter 'fuse'

Reported-by: Qing Wang <qinwang@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210816180442.2000642-1-philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>