rust/vmstate: Support vmstate_validate

rust/vmstate: Support vmstate_validate

In C version, VMSTATE_VALIDATE accepts the function pointer, which is
used to check if some conditions of structure could meet, although the
C version macro doesn't accept any structure as the opaque type.

But it's hard to integrate VMSTATE_VALIDAE into vmstate_struct, a new
macro has to be introduced to specifically handle the case corresponding
to VMSTATE_VALIDATE.

One of the difficulties is inferring the type of a callback by its name
`test_fn`. We can't directly use `test_fn` as a parameter of
test_cb_builder__() to get its type "F", because in this way, Rust
compiler will be too conservative on drop check and complain "the
destructor for this type cannot be evaluated in constant functions".

Fortunately, PhantomData<T> could help in this case, because it is
considered to never have a destructor, no matter its field type [*].

The `phantom__()` in the `call_func_with_field` macro provides a good
example of using PhantomData to infer type. So copy this idea and apply
it to the `vmstate_validate` macro.

[*]: https://doc.rust-lang.org/std/ops/trait.Drop.html#drop-check

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-11-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>