This commit changes guestfs_launch so that it both launches
the appliance and waits until it is ready (ie. the daemon communicates
back to us).

Since we removed the pretence that we could implement a low-level
asynchronous API, the need to call launch() followed by wait_ready()
has looked a bit silly.

Now guestfs_wait_ready() is basically a no-op.  It is left in the
API for backwards compatibility.  Any calls to guestfs_wait_ready()
can be removed from client code.