rust/vmstate: Fix size field of VMStateField with VMS_ARRAY_OF_POINTER flag

rust/vmstate: Fix size field of VMStateField with VMS_ARRAY_OF_POINTER flag

The `size` field of the VMStateField with VMS_ARRAY_OF_POINTER flag
should stores the size of pointer, which depends on platform.

Currently, `*const`, `*mut`, `NonNull`, `Box<>` and their wrapper are
supported, and they have the same size as `usize`.

Store the size (of `usize`) when VMS_ARRAY_OF_POINTER flag is set.

The size may be changed when more smart pointers are supported, but now
the size of "usize" is enough.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-5-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>