fuse: alloc initial fuse_conn and fuse_mount

In fuse and virtiofs these are already allocated, so move the allocation in
the common initialization helper.

Also change cuse so that these objects are allocated separately, as opposed
to having them inline in cuse_conn.  This will aid in cleaning up the
exported fuse transport interface.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>