block/vdi: 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 vdi_co_do_create() passes @errp to error_prepend() without
ERRP_GUARD(), and its @errp parameter is so widely sourced that it is
necessary to protect it with ERRP_GUARD().

To avoid the potential issues as [1] said, 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: Stefan Weil <sw@weilnetz.de>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Message-ID: <20240311033822.3142585-12-zhao1.liu@linux.intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

block/vdi.c

@@ -738,6 +738,7 @@ static int coroutine_fn GRAPH_UNLOCKED
 vdi_co_do_create(BlockdevCreateOptions *create_options, size_t block_size,
                  Error **errp)
 {
+    ERRP_GUARD();
     BlockdevCreateOptionsVdi *vdi_opts;
     int ret = 0;
     uint64_t bytes = 0;