fuse: In mount-local-run, test if root filesystem has been mounted (RHBZ#1057504).

It is never normally valid to use the mount-local* APIs when you
haven't mounted some filesystems in the libguestfs namespace.

If you try it, it results in some odd errors.  The mount-local-run
call is successful, but subsequent operations fail:

$ mkdir -p /tmp/mnt
$ guestfish -x -N fs mount-local /tmp/mnt : mount-local-run
libguestfs: error: lstat: lstat_stub: you must call 'mount' first to mount the root filesystem
libguestfs: error: lstat: lstat_stub: you must call 'mount' first to mount the root filesystem

With this commit, the mount-local-run call gives an error:

libguestfs: error: you must call 'guestfs_mount' first to mount a filesystem on '/'.
Note: '/tmp/mnt' is still mounted.  Use 'guestunmount /tmp/mnt' to clean up.

The error is implemented by calling guestfs_exists (g, "/") which has
the side effect of running the NEED_ROOT macro in the daemon.  (There
is no simple equivalent of NEED_ROOT on the library side.)