block: Check blockers in all nodes involved in a block-commit job

qmp_block_commit() checks for op blockers in the active and
destination (base) images. However all nodes between top_bs and base
are also involved, and they are removed from the chain afterwards.

In addition to that, if top_bs is not the active layer then top_bs's
overlay also needs to be checked because it's involved in the job (its
backing image string needs to be updated to point to 'base').

This patch checks that none of those nodes are blocked.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
