backends: Have HostMemoryBackendClass::alloc() handler return a boolean

backends: Have HostMemoryBackendClass::alloc() handler return a boolean

Following the example documented since commit e3fe3988d7 ("error:
Document Error API usage rules"), have HostMemoryBackendClass::alloc
return a boolean indicating whether an error is set or not.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20231120213301.24349-17-philmd@linaro.org>