monitor: Create monitor-internal.h with common definitions

monitor: Create monitor-internal.h with common definitions

Before we can split monitor/misc.c, we need to create a header file that
contains the common definitions that will be used by multiple source
files.

For a start, add the type definitions for Monitor, MonitorHMP and
MonitorQMP and their dependencies. We'll add functions as needed when
splitting monitor/misc.c.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190613153405.24769-10-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Header guard symbol tidied up, superfluous #include dropped, FIXME in
hmp_change() resolved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>