xapi: report a clear error when host evacuation is blocked by unprotected VMs

[olivierlambert]

Why

When HA is enabled on a pool, host.evacuate (and host.restart, which calls
it) 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). The change is a pure diagnostic
improvement with no change to which VMs can be evacuated, so no new automated
test is included.

Closes #4323

[minglumlu]

It's nice to get rid of the long standing confusing error message.

[gthvn1]

LGTM. I only don't understand what is the host.restart in the commit message. I don't see such API in XAPI. Is it something higher level? I would consider removing that reference.

[psafont]

Suggested change

	"The host cannot be evacuated because HA is enabled on the pool and the \
	VM is not HA-protected (its ha_restart_priority is not set to \
	"The host cannot be evacuated because HA is enabled on the pool and a VM \
	running on it is not HA-protected (its ha_restart_priority is not set to \