blockdev: Rename BlockdevAction -> TransactionAction

There's no reason to restrict transactions to operations related to
block devices, so rename the type now before schema introspection stops
us from doing so.

Also change the schema documentation of 'transaction' to not refer to
block devices or snapshots any more.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>