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

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

Out-of-band command execution was introduced in commit cf869d53172.
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 951702f39c7 "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 <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20181009062718[email protected]>
[Conflict with "monitor: check if chardev can switch gcontext for OOB"
resolved, commit message updated]
Signed-off-by: Markus Armbruster <[email protected]>