qemu: checkpoint: Introduce helper to find checkpoint disk definition in parents

qemu: checkpoint: Introduce helper to find checkpoint disk definition in parents

The algorithm is used in two places to find the parent checkpoint object
which contains given disk and then uses data from the disk. Additionally
the code is written in a very non-obvious way. Factor out the lookup of
the disk into a function which also simplifies the callers.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>