qapi: Rename WatchdogExpirationAction enum

The new name is WatchdogAction which is shorter,

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <dbd61a0928821348486d0d6260be2bd3b02b6402.1504771369.git.mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>