Make the readline.c functionality reusable.  Instead of calling
monitor_printf() and monitor_flush() directly, invoke function pointers
provided by the user.

This way readline.c does not know about Monitor and other users will be
able to make use of readline.c.

Note that there is already an "opaque" argument to the ReadLineFunc
callback.  Consistently call it "readline_opaque" from now on to
distinguish from the ReadLinePrintfFunc/ReadLineFlushFunc "opaque"
argument.

I also dropped the printf macro trickery since it's now highly unlikely
that anyone modifying readline.c would call printf(3) directly.  We no
longer need this protection.

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