monitor: Collect "control" command handlers in qmp-cmds.control.c

monitor: Collect "control" command handlers in qmp-cmds.control.c

Move all of the QMP commands handlers to implement the 'control' module
(qapi/control.json) that can be shared between the system emulator and
tools such as a storage daemon to a new file monitor/qmp-cmds-control.c.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200129102239.31435-4-kwolf@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>