The virStorageFileLoadBackendModule method is only used if either
fs or gluster storage is built in, which doesn't happen on mingw
leading to warning of an unused static function.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

The storage file drivers are currently loaded as a side effect of
loading the storage driver. This is a bogus dependancy because the
storage file code has no interaction with the storage drivers, and
even ultimately be running in a completely separate daemon.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

The QEMU driver loadable module needs to be able to resolve all ELF
symbols it references against libvirt.so. Some of its symbols can only
be resolved against the storage_driver.so loadable module which creates
a hard dependancy between them. By moving the storage file backend
framework into the util directory, this gets included directly in the
libvirt.so library. The actual backend implementations are still done as
loadable modules, so this doesn't re-add deps on gluster libraries.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

The storage driver backends are serving the public storage pools API,
while the storage file backends are serving the internal QEMU driver and
/ or libvirt utility code.

To prep for moving this storage file backend framework into the utility
code, split out the backend definitions.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>