util/oslib: Have qemu_prealloc_mem() handler return a boolean

util/oslib: Have qemu_prealloc_mem() handler return a boolean

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

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20231120213301.24349-19-philmd@linaro.org>