Add a section that outlines usage of tools to handle bitmaps and
introduce terms corresponding to the output of qemu-img to be used in
further sections.

With this we can simplify the section about checking bitmap health as we
don't have to explain the qemu-img output but can refer to the newly
defined terms.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>