python: Treat None-return of greeting cmd

python: Treat None-return of greeting cmd

In case qemu process dies the "monitor.cmd" returns None which gets
passed to the "__negotiate_capabilities" and leads to unhandled
exception. Let's only check the resp in case it has a value.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200120071202.30646-1-ldoktor@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>