block/qcow2: Fix missing ERRP_GUARD() for error_prepend()

block/qcow2: 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/qcow2.c, there are 2 functions passing @errp to error_prepend()
without ERRP_GUARD():
 - qcow2_co_create()
 - qcow2_co_truncate()

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

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

[1]: Issue description in the commit message of commit ae7c80a7bd73
     ("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>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20240311033822.3142585-9-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>