storage: extract storage file backend from main storage driver backend

storage: extract storage file backend from main storage driver backend

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: Daniel P. Berrange <berrange@redhat.com>