qemu: block: Convert disk 'storage' backend JSON props generator to new accessors

qemu: block: Convert disk 'storage' backend JSON props generator to new accessors

We need to use the 'effective' storage nodename (one which includes the
optional storage slice 'raw' intermediate layer) in the code which
formats the 'format' layer props.

All other cases need the real storage driver nodename as they either
generate the 'storage' layer props, or the storage slice, which refers
to the proper storage backend.

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