It's probably a hack, but this usually works for me:
# Make sure VM is disabled:
ha-manager set vm:<VMID> --state disabled
# Open GDISK to modify disk partition map
gdisk /dev/zvol/rpool/vm-<VMID>-disk-<DISK#>
# Once GDISK opens, then just use the W command to re-write the partiion map
#...