monitor: expose readline state

HMP is now implemented in terms of QMP.  The monitor has a bunch of logic to
deal with HMP right now like readline support.  Export it from the monitor so
we can consume it in hmp.c.

In short time, hmp.c will take over all of the readline bits.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
