hw/nubus/virtio-mmio: Fix missing ERRP_GUARD() in realize handler

hw/nubus/virtio-mmio: Fix missing ERRP_GUARD() in realize handler

According to the comment in qapi/error.h, dereferencing @errp requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
* - It must not be dereferenced, because it may be null.
...
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.
*
* Using it when it's not needed is safe, but please avoid cluttering
* the source with useless code.

In nubus_virtio_mmio_realize(), @errp is dereferenced without
ERRP_GUARD().

Although nubus_virtio_mmio_realize() - as a DeviceClass.realize()
method - is never passed a null @errp argument, it should follow the
rules on @errp usage.  Add the ERRP_GUARD() there.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240723161802.1377985-1-zhao1.liu@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>