block/nvme: 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 nvme.c, there are 3 functions passing @errp to error_prepend()
without ERRP_GUARD():
- nvme_init_queue()
- nvme_create_queue_pair()
- nvme_identify()

All these 3 functions take their @errp parameters from the
nvme_file_open(), which is a BlockDriver.bdrv_nvme() method and its
@errp points to its caller's local_err.

Though these 3 cases haven't trigger the issue like [1] said, to
follow the requirement of @errp, add missing ERRP_GUARD() at their
beginning.

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

Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Fam Zheng <fam@euphon.net>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
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: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240311033822.3142585-7-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

block/nvme.c

@@ -168,6 +168,7 @@ static QemuOptsList runtime_opts = {
 static bool nvme_init_queue(BDRVNVMeState *s, NVMeQueue *q,
                             unsigned nentries, size_t entry_bytes, Error **errp)
 {
+    ERRP_GUARD();
     size_t bytes;
     int r;
 
@@ -221,6 +222,7 @@ static NVMeQueuePair *nvme_create_queue_pair(BDRVNVMeState *s,
                                              unsigned idx, size_t size,
                                              Error **errp)
 {
+    ERRP_GUARD();
     int i, r;
     NVMeQueuePair *q;
     uint64_t prp_list_iova;
@@ -535,6 +537,7 @@ static int nvme_admin_cmd_sync(BlockDriverState *bs, NvmeCmd *cmd)
 /* Returns true on success, false on failure. */
 static bool nvme_identify(BlockDriverState *bs, int namespace, Error **errp)
 {
+    ERRP_GUARD();
     BDRVNVMeState *s = bs->opaque;
     bool ret = false;
     QEMU_AUTO_VFREE union {