rust: vmstate: add new type safe implementation

rust: vmstate: add new type safe implementation

The existing translation of the C macros for vmstate does not make
any attempt to type-check vmstate declarations against the struct, so
introduce a new system that computes VMStateField based on the actual
struct declaration.

Macros do not have full access to the type system, therefore a full
implementation of this scheme requires a helper trait to analyze the
type and produce a VMStateField from it; a macro "vmstate_of!" accepts
arguments similar to "offset_of!" and tricks the compiler into looking
up the trait for the right type.

The patch introduces not just vmstate_of!, but also the slightly too
clever enabling macro call_func_with_field!.  The particular trick used
here was proposed on the users.rust-lang.org forum, so I take no merit
and all the blame.

Introduce the trait and some functions to access it; the actual
implementation comes later.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>