rust: declare self as qemu_api for proc-macros

rust: declare self as qemu_api for proc-macros

Fix an outstanding TODO.

Declaring `extern crate self as qemu_api` allows use of `qemu_api`
within the qemu_api crate; this allows the Wrapper derive macro and
future proc macros to be used interchangeably in the qemu_api crate and
other crates. This is not required currently and is only for
future-proofing.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/20250728-self-as-qemu_api-v1-1-001c339cccc8@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>