rust: vmstate: use const_refs_to_static

rust: vmstate: use const_refs_to_static

The VMStateDescriptionBuilder already needs const_refs_static, so
use it to remove the need for vmstate_clock! and vmstate_struct!,
as well as to simplify the implementation for scalars.

If the consts in the VMState trait can reference to static
VMStateDescription, scalars do not need the info_enum_to_ref!
indirection and structs can implement the VMState trait themselves.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250908105005.2119297-9-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>