hw/vfio/helpers: 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].

In hw/vfio/helpers.c, there are 3 functions passing @errp to
error_prepend() without ERRP_GUARD():
 - vfio_set_irq_signaling()
 - vfio_device_get_name()
 - vfio_device_set_fd()

There are too many possible callers to check the impact of this defect;
it may or may not be harmless. Thus it is necessary to protect their
@errp with ERRP_GUARD().

To avoid the issue like [1] said, add missing ERRP_GUARD() at their
beginning.

[1]: Issue description in the commit message of commit ae7c80a
     ("error: New macro ERRP_GUARD()").

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-ID: <20240311033822.3142585-21-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/vfio/helpers.c

@@ -110,6 +110,7 @@ static const char *index_to_str(VFIODevice *vbasedev, int index)
 int vfio_set_irq_signaling(VFIODevice *vbasedev, int index, int subindex,
                            int action, int fd, Error **errp)
 {
+    ERRP_GUARD();
     struct vfio_irq_set *irq_set;
     int argsz, ret = 0;
     const char *name;
@@ -613,6 +614,7 @@ bool vfio_has_region_cap(VFIODevice *vbasedev, int region, uint16_t cap_type)
 
 int vfio_device_get_name(VFIODevice *vbasedev, Error **errp)
 {
+    ERRP_GUARD();
     struct stat st;
 
     if (vbasedev->fd < 0) {
@@ -644,6 +646,7 @@ int vfio_device_get_name(VFIODevice *vbasedev, Error **errp)
 
 void vfio_device_set_fd(VFIODevice *vbasedev, const char *str, Error **errp)
 {
+    ERRP_GUARD();
     int fd = monitor_fd_param(monitor_cur(), str, errp);
 
     if (fd < 0) {