backends: Use g_autofree in HostMemoryBackendClass::alloc() handlers

backends: Use g_autofree in HostMemoryBackendClass::alloc() handlers

In preparation of having HostMemoryBackendClass::alloc() handlers
return a boolean, have them use g_autofree.

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: <20231120213301.24349-15-philmd@linaro.org>