qapi: Tidy up examples

A few examples neglect to prefix QMP input with '->'.  Fix that.

Two examples have extra space after '<-'.  Delete it.

A few examples neglect to show output.  Provide some.  The example
output for query-vcpu-dirty-limit could use further improvement.  Add
a TODO comment.

Use "Examples:" instead of "Example:" where multiple examples are
given.

One example section numbers its two examples.  Not done elsewhere;
drop.

Another example section separates them with "or".  Likewise.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-8-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

qapi/char.json

@@ -637,7 +637,7 @@
 #
 # Since: 1.4
 #
-# Example:
+# Examples:
 #
 # -> { "execute" : "chardev-add",
 #      "arguments" : { "id" : "foo",
@@ -673,7 +673,7 @@
 #
 # Since: 2.10
 #
-# Example:
+# Examples:
 #
 # -> { "execute" : "chardev-change",
 #      "arguments" : { "id" : "baz",