qemu: domain: Convert the status XML code for 'storage' nodenames to new accessors

qemu: domain: Convert the status XML code for 'storage' nodenames to new accessors

Use the new accessors in the private XML formatters and parsers and the
recovery code.

Specifically in all instances we use the proper (not effective) storage
nodename. In the virStorageSource private data it is what we need to
store. In blockjobs status XML it simply serves us to find the
appropriate 'virStorageSource' struct so using the storage layer node
name is simpler.

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