rust: vmstate: implement VMState for scalar types

rust: vmstate: implement VMState for scalar types

Scalar types are those that have their own VMStateInfo.  This poses
a problem in that references to VMStateInfo can only be included in
associated consts starting with Rust 1.83.0, when the const_refs_static
was stabilized.  Removing the requirement is done by placing a limited
list of VMStateInfos in an enum, and going from enum to &VMStateInfo
only when building the VMStateField.

The same thing cannot be done with VMS_STRUCT because the set of
VMStateDescriptions extends to structs defined by the devices.
Therefore, structs and cells cannot yet use vmstate_of!.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>