monitor: Fix unsafe sharing of @cur_mon among threads

monitor: Fix unsafe sharing of @cur_mon among threads

@cur_mon is null unless the main thread is running monitor code, either
HMP code within monitor_read(), or QMP code within
monitor_qmp_dispatch().

Use of @cur_mon outside the main thread is therefore unsafe.

Most of its uses are in monitor command handlers.  These run in the main
thread.

However, there are also uses hiding elsewhere, such as in
error_vprintf(), and thus error_report(), making these functions unsafe
outside the main thread.  No such unsafe uses are known at this time.
Regardless, this is an unnecessary trap.  It's an ancient trap, though.

More recently, commit cf869d53172 "qmp: support out-of-band (oob)
execution" spiced things up: the monitor I/O thread assigns to @cur_mon
when executing commands out-of-band.  Having two threads save, set and
restore @cur_mon without synchronization is definitely unsafe.  We can
end up with @cur_mon null while the main thread runs monitor code, or
non-null while it runs non-monitor code.

We could fix this by making the I/O thread not mess with @cur_mon, but
that would leave the trap armed and ready.

Instead, make @cur_mon thread-local.  It's now reliably null unless the
thread is running monitor code.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[peterx: update subject and commit message written by Markus]
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180720033451.32710-1-peterx@redhat.com>