storage_file: de-modularize the local file backend

storage_file: de-modularize the local file backend

The 'storage_file' infrastructure serves as an abstraction on top of
file-looking storage technologies. Apart from local file it currently
implements also a backend for 'gluster'.

Historically it was all modularized and the local file module was
usually packaged with the 'core' part of the storage driver. Now with
split daemons one can install e.g. 'virqemud' without the storage driver
core which contains the 'fs' backend module. Since the qemu driver uses
the storage file backends to e.g. create storage for snapshots and
backups this allows users to create a deployment where some things will
not work properly.

As the 'fs' backend doesn't use any code that wouldn't be linked
directly anyways there's no point in actually shipping it as a module.

Let's compile it in so that all deployments can use it.

To achieve that, compile the source directly into the
'virt_storage_file_lib' static library and remove the loading code. Also
adjust the spec file.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>