The ReadLineState in Monitor is only used for HMP monitors. Create
MonitorHMP and move it there.

Can't use container_of() in hmp_change().  Cast instead, and mark
FIXME.  Will be cleaned up shortly.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190613153405.24769-5-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Superfluous variable in monitor_data_destroy() eliminated, whitespace
tweaked in hmp_change(), commit message improved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>