qemu: domain: Introduce helper to convert <loader> into virStorageSource

qemu: domain: Introduce helper to convert <loader> into virStorageSource

Add a helper which will covert the PFLASH code file and variable file
into the virStorageSource objects stored in private data so that we can
use them with -blockdev while keeping the infrastructure to determine
the path to the loaders intact.

This is a temporary solution until we will want to do snapshots of the
pflash where we will be forced do track the full backing chain in the
XML.

In the meanwhile just convert it partially so that we can stop using
-drive.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>