watchdog: Consolidate QAPI into single file

Commit f0df84c6 added watchdog-set-action in the main qapi-schema.json,
but it belongs better in qapi/run-state.json alongside the definition
of WatchdogAction.  The command was written prior to commit 0e201d34
creating the latter file, even though it was merged after.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180226225744.26356-1-eblake@redhat.com>