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xapi: report a clear error when host evacuation is blocked by unprotected VMs (#7145)

## Why

When HA is enabled on a pool, `host.evacuate` refuses to plan the
evacuation of
any VM that is not HA-protected, that is, whose `ha_restart_priority` is
not
`restart`. This is by design: the HA planner only accounts for protected
VMs, so
unprotected ones are left out of the plan.

The problem is the error reported in that situation.
`compute_evacuation_plan_no_wlb`
marked every unprotected VM with `HOST_NOT_ENOUGH_FREE_MEMORY`, and
`host.evacuate`
then raised it. That error is misleading in two ways:

* It blames free memory, so operators investigate RAM on the destination
hosts
  (which is usually plentiful) instead of the real cause.
* It is raised with a single parameter (the VM reference), while
`HOST_NOT_ENOUGH_FREE_MEMORY` is documented as taking `[needed;
available]`.
  Clients such as Xen Orchestra therefore render the available memory as
  `<unknown>`.

This has confused users for years. See #4323 and the forum reports
linked from
it: evacuation fails with `HOST_NOT_ENOUGH_FREE_MEMORY` even when the
destination
has tens of GB free, purely because a resident VM is not HA-protected.

## How

Introduce a dedicated error, `HOST_EVACUATE_VM_NOT_HA_PROTECTED`,
carrying the VM
reference, and raise it instead for unprotected VMs. The message states
the real
cause and the ways to resolve it (set the VM's `ha_restart_priority` to
`restart`, shut down or suspend the VM, or disable HA before evacuating
the
host).

Behaviour is otherwise unchanged: the evacuation still fails for these
VMs, it is
just reported correctly.

Files touched:

* `ocaml/xapi-consts/api_errors.ml`: declare the error.
* `ocaml/idl/datamodel_errors.ml`: register it with a description.
* `ocaml/xapi/xapi_host.ml`: raise it for unprotected VMs in the
evacuation plan.

## Testing

Built and tested on CI (build and test, unit tests, format, CodeChecker
and
ShellCheck all green on a fork branch... (continued)

3667 of 4642 relevant lines covered (79.0%)

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