govc: adding RDM mapping stored in VMDK fails when using vm.disk.attach and vm.rdm.attach does not provide vmdk option

[etosan]

Describe the bug
When adding RDM encoded in VMDK mapping file, or so called "Physical Compatibility Mode Raw Device Mapping" disk, the operation fails with error: Incompatible device backing specified for device '0'.

To Reproduce
Create RDM as VMDK and try to attach it as VMDK disk:

1. Create LUN on SAN storage system
2. Refresh the hosts and rescan the storage systems through vsphere WebGUI to get LUN address
3. Create Physical Compatibility Mode Raw Device Mapping as discussed here to get situation like here
4. Try to add resulting vmdk using govc like this: govc vm.disk.attach -vm dbcluster-c01.infra.environment -ds ds-c01-rdm01 -disk ds-c01-dbcluster-sbd-1.vmdk -link=false
5. Operation fails with error: Incompatible device backing specified for device '0'.

Expected behavior
RDM VMDK disk should be added as normal. Adding said VMDK works fine through vsphere webgui.

Affected version
govc does not provide -version flag so I don't know what to report, binary is sufficiently recent, but I have no problem to retry with newest version if instructed.

Screenshots/Debug Output
N/A

Additional context
Same issue, but misidentified, recorded here. Reporter does not realize there is difference between raw LUN based RDM and RDM "symlinks"/mapping files.
Affects terraform provider here as well, although it has other issues not directly caused by this BUG.

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