rust: allow older version of bindgen

rust: allow older version of bindgen

Cope with the old version that is provided in Debian 12.

--size_t-is-usize is needed on bindgen <0.61.0, and it was removed in
bindgen 0.65.0, so check for it in meson.build.

--merge-extern-blocks was added in 0.61.0.

--formatter rustfmt was added in 0.65.0 and is the default, so remove it.

Apart from Debian 12 and Ubuntu 22.04, all other supported distros have
version 0.66.x of bindgen or newer (or do not have bindgen at all).

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