From what I checked the iohyve puts disk images as ZFS zvol volumes like:

- /dev/zvol/zroot/iohyve/VMNAME/disk0

So the migration would be:

- create your new VM with disk of the same size

- check your new VM disk place at 'vm-bhyve':

    # vm info VM | grep system-path 
    system-path: /vm/VM/disk0.img

- copy contents of 'iohyve' disk into VM disk

    # dd bs=1m if=/dev/zvol/zroot/iohyve/VMNAME/disk0 of=/vm/VM/disk0.img

- start your new VM

This should work.

Same with libvirt/virt-manager - just copy your VM image from /var/lib/libvirt/images/ dir and it should work the same.

Hope that helps.

Hope that helps.