Add a validator which checks that a bitmap spanning multiple backing
chain members doesn't look broken. The current rules are that no
intermediate birmaps are missing (unfortunately it's hard to know
whether the topmost or bottommost bitmap is missing) and none of the
components is inconsistent.

We can obviously improve it over time.

The validator is also tested against the existing bitmap data we have
for the backup merging test as well as some of the existing broken
bitmap synthetic test cases.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>