net/vhost-vdpa: 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 net_init_vhost_vdpa() passes @errp to error_prepend(), and as a
member of net_client_init_fun[], it's called in net_client_init1() and
gets @errp from this caller.

But because netdev_init_modern() passes &error_fatal to
net_client_init1(), then @errp parameter of net_init_vhost_vdpa() would
point to @error_fatal. This causes the error message in error_prepend()
to be lost because of the above issue.

To fix this, add missing ERRP_GUARD() at the beginning of this function.

[1]: Issue description in the commit message of commit ae7c80a7bd
     ("error: New macro ERRP_GUARD()").

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240311033822.3142585-29-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

net/vhost-vdpa.c

@@ -1751,6 +1751,7 @@ static int vhost_vdpa_get_max_queue_pairs(int fd, uint64_t features,
 int net_init_vhost_vdpa(const Netdev *netdev, const char *name,
                         NetClientState *peer, Error **errp)
 {
+    ERRP_GUARD();
     const NetdevVhostVDPAOptions *opts;
     uint64_t features;
     int vdpa_device_fd;