qapi: Add a 'coroutine' flag for commands

This patch adds a new 'coroutine' flag to QMP command definitions that
tells the QMP dispatcher that the command handler is safe to be run in a
coroutine.

The documentation of the new flag pretends that this flag is already
used as intended, which it isn't yet after this patch. We'll implement
this in another patch in this series.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201005155855.256490-9-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>