Use qemuBlockBitmapsHandleBlockcopy to calculate bitmaps to copy over
for a block-copy job.

We copy them when pivoting to the new image as at that point we are
certain that we don't dirty any bitmap unnecessarily.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>