The <mirror> subelement is used in two ways: in a commit job to point to
existing storage, and in a block-copy job to point to additional
storage. We need a way to track only the distinct storage.

This patch introduces qemuBlockJobDiskRegisterMirror which registers the
mirror chain separately only for jobs which require it. This also comes
with remembering that in the status XML.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
ACKed-by: NEric Blake <eblake@redhat.com>