backends/iommufd: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* 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.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

The iommufd_backend_set_fd() passes @errp to error_prepend(), to avoid
the above issue, add missing ERRP_GUARD() at the beginning of this
function.

[1]: Issue description in the commit message of commit ae7c80a7bd73
     ("error: New macro ERRP_GUARD()").

Cc: Yi Liu <yi.l.liu@intel.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Zhenzhong Duan <zhenzhong.duan@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-ID: <20240311033822.3142585-3-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

backends/iommufd.c

@@ -43,6 +43,7 @@ static void iommufd_backend_finalize(Object *obj)
 
 static void iommufd_backend_set_fd(Object *obj, const char *str, Error **errp)
 {
+    ERRP_GUARD();
     IOMMUFDBackend *be = IOMMUFD_BACKEND(obj);
     int fd = -1;