hw/intc/arm_gicv3_its: Don't abort on table save failure

The ITS is not fully properly reset at the moment. Caches are
not emptied.

After a reset, in case we attempt to save the state before
the bound devices have registered their MSIs and after the
1st level table has been allocated by the ITS driver
(device BASER is valid), the first level entries are still
invalid. If the device cache is not empty (devices registered
before the reset), vgic_its_save_device_tables fails with -EINVAL.
This causes a QEMU abort().

Cc: qemu-stable@nongnu.org
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reported-by: wanghaibin <wanghaibin.wang@huawei.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
