target/i386/sev: 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 the pointer of
error_fatal, the user can't see this additional information, because
exit() happens in error_setg earlier than information is added [1].

The sev_inject_launch_secret() passes @errp to error_prepend(), and as
an APIs defined in target/i386/sev.h, it is necessary to protect its
@errp with ERRP_GUARD().

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

[1]: Issue description in the commit message of commit ae7c80a7bd73
     ("error: New macro ERRP_GUARD()").

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20240229143914.1977550-17-zhao1.liu@linux.intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

target/i386/sev.c

@@ -1044,6 +1044,7 @@ sev_encrypt_flash(uint8_t *ptr, uint64_t len, Error **errp)
 int sev_inject_launch_secret(const char *packet_hdr, const char *secret,
                              uint64_t gpa, Error **errp)
 {
+    ERRP_GUARD();
     struct kvm_sev_launch_secret input;
     g_autofree guchar *data = NULL, *hdr = NULL;
     int error, ret = 1;