Struct qemuDomainDiskPrivate was holding multiple variables connected to
a disk block job. Consolidate them into a new struct qemuBlockJobData.

This will also allow simpler extensions to the block job mechanisms.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>