rust/zeroable: Implement Zeroable with const_zero macro

rust/zeroable: Implement Zeroable with const_zero macro

The `const_zero` crate provides a nice macro to zero type-specific
constants, which doesn't need to enumerates the fields one by one.

Introduce the `const_zero` macro to QEMU (along with its documentation), and
use it to simplify the implementation of `Zeroable` trait.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250123163143.679841-1-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>