qapi block: Elide redundant has_FOO in generated C

The has_FOO for pointer-valued FOO are redundant, except for arrays.
They are also a nuisance to work with.  Recent commit "qapi: Start to
elide redundant has_FOO in generated C" provided the means to elide
them step by step.  This is the step for qapi/block*.json.

Said commit explains the transformation in more detail.

There is one instance of the invariant violation mentioned there:
qcow2_signal_corruption() passes false, "" when node_name is an empty
string.  Take care to pass NULL then.

The previous two commits cleaned up two more.

Additionally, helper bdrv_latency_histogram_stats() loses its output
parameters and returns a value instead.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221104160712.3005652-11-armbru@redhat.com>
[Fixes for #ifndef LIBRBD_SUPPORTS_ENCRYPTION and MacOS squashed in]

qemu-nbd.c

@@ -1107,9 +1107,7 @@ int main(int argc, char **argv)
         .has_writable       = true,
         .writable           = !readonly,
         .u.nbd = {
-            .has_name             = true,
             .name                 = g_strdup(export_name),
-            .has_description      = !!export_description,
             .description          = g_strdup(export_description),
             .has_bitmaps          = !!bitmaps,
             .bitmaps              = bitmaps,