Similarly to how we've intergrated data belonging to a single
virStorageSource for purposes of attaching it to a qemu instance we will
need to agregate data relevant for the whole disk. With blockdev there
will be some disk-wide backing chain members such as the copy-on-read
handler.

Introduce qemuHotplugDiskSourceData which agregates the backing chain
and other data relevant for the disk and functions which generate it
and apply and rollback it.

In addition to disk hotplug this will also be reused for media changing
where we need to exchange the full disk backend.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>