monitor: Remove "x-oob", offer capability "oob" unconditionally

Out-of-band command execution was introduced in commit cf869d5317.
Unfortunately, we ran into a regression, and had to turn it into an
experimental option for 2.12 (commit be933ffc23).

  http://lists.gnu.org/archive/html/qemu-devel/2018-03/msg06231.html

The regression has since been fixed (commit 951702f39c "monitor: bind
dispatch bh to iohandler context").  A thorough re-review of OOB
commands led to a few more issues, which have also been addressed.

This patch partly reverts be933ffc23 (monitor: new parameter "x-oob"),
and makes QMP monitors again offer capability "oob" whenever they can
provide it, i.e. when the monitor's character device is capable of
running in an I/O thread.

Some trivial touch-up in the test code is required to make sure qmp-test
won't break.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20181009062718.1914-4-peterx@redhat.com>
[Conflict with "monitor: check if chardev can switch gcontext for OOB"
resolved, commit message updated]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

tests/qmp-test.c

@@ -116,7 +116,7 @@ static void test_qmp_protocol(void)
     g_assert(q);
     test_version(qdict_get(q, "version"));
     capabilities = qdict_get_qlist(q, "capabilities");
-    g_assert(capabilities && qlist_empty(capabilities));
+    g_assert(capabilities);
     qobject_unref(resp);
 
     /* Test valid command before handshake */