hw/vfio/iommufd: Fix missing ERRP_GUARD() in iommufd_cdev_getfd()

hw/vfio/iommufd: Fix missing ERRP_GUARD() in iommufd_cdev_getfd()

As the comment in qapi/error, 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.

But in iommufd_cdev_getfd(), @errp is dereferenced without ERRP_GUARD():

if (*errp) {
    error_prepend(errp, VFIO_MSG_PREFIX, path);
}

Currently, since vfio_attach_device() - the caller of
iommufd_cdev_getfd() - is always called in DeviceClass.realize() context
and doesn't get the NULL @errp parameter, iommufd_cdev_getfd()
hasn't triggered the bug that dereferencing the NULL @errp.

To follow the requirement of @errp, add missing ERRP_GUARD() in
iommufd_cdev_getfd().

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20240223085653.1255438-7-zhao1.liu@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>