rust: let bilge use "let ... else"

rust: let bilge use "let ... else"

"let ... else" was stabilized in 1.65.0; bumping the minimum supported
Rust version means we don't need to patch it out anymore.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>