Virtual Machine (VM) migration presents several advantages for both cloud operators and cloud users. The benefits of VM migration are amplified in the context of Jointclouds, in which multiple clouds owned by distinct operators exchange VMs. However, inter-cloud VM migrations might require the usage of the Internet, which creates the possibility of external threats and therefore, a risk to the VMs' integrity. Thus, we propose a novel approach to secure VM migration for Joint-clouds. We consider both local migration and inter-cloud migration. Our solution is based on a multi-blockchain system, in which each cloud provider participates in the maintenance of a consortium blockchain to store relevant information about the migrations. In addition, each cloud operator might choose to keep a local blockchain to secure its local VM migrations. Therefore, inter-blockchain communication plays an important role in our work.