Now that the monitor and readline are decoupled, readline.h no longer
belongs in include/monitor/.  Put the header into include/qemu/.

Move the source file into util/ so it can be linked as part of
libqemuutil.a.

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