accel/kvm: Silent -Wmissing-field-initializers warning

Silent when compiling with -Wextra:

  ../accel/kvm/kvm-all.c:2291:17: warning: missing field 'num' initializer [-Wmissing-field-initializers]
        { NULL, }
                ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221220143532.24958-3-philmd@linaro.org>