rust: do not use MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy but is not available as a "const" function
until Rust 1.75.0.

Remove the default implementation of Zeroable::ZERO, and write by

rust: do not use MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy but is not available as a "const" function
until Rust 1.75.0.

Remove the default implementation of Zeroable::ZERO, and write by hand
the definitions for those types that need it. It may be possible to
add automatic implementation of the trait, via a procedural macro and/or
a trick similar to offset_of!, but do it the easy way for now.

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

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rust: provide safe wrapper for MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy, but it introduces unsafe (and has a
pretty heavy syntax in general). Introduce a trait that provides the
same fu

rust: provide safe wrapper for MaybeUninit::zeroed()

MaybeUninit::zeroed() is handy, but it introduces unsafe (and has a
pretty heavy syntax in general). Introduce a trait that provides the
same functionality while staying within safe Rust.

In addition, MaybeUninit::zeroed() is not available as a "const"
function until Rust 1.75.0, so this also prepares for having handwritten
implementations of the trait until we can assume that version.

Reviewed-by: Junjie Mao <junjie.mao@hotmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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