block/snapshot: 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 block/snapshot.c, there are 2 functions passing @errp to
error_prepend() without ERRP_GUARD():
 - bdrv_all_delete_snapshot()
 - bdrv_all_goto_snapshot()

As the APIs exposed in include/block/snapshot.h, they could be called
by other modules.

To avoid potential issues as [1] said, add missing ERRP_GUARD() at the
beginning of these 2 functions.

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

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: <20240311033822.3142585-11-zhao1.liu@linux.intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

block/snapshot.c

@@ -566,6 +566,7 @@ int bdrv_all_delete_snapshot(const char *name,
                              bool has_devices, strList *devices,
                              Error **errp)
 {
+    ERRP_GUARD();
     g_autoptr(GList) bdrvs = NULL;
     GList *iterbdrvs;
 
@@ -605,6 +606,7 @@ int bdrv_all_goto_snapshot(const char *name,
                            bool has_devices, strList *devices,
                            Error **errp)
 {
+    ERRP_GUARD();
     g_autoptr(GList) bdrvs = NULL;
     GList *iterbdrvs;
     int ret;