monitor: tighten monitor_printf*()

Rename monitor_printf->monitor_hmp_printf, monitor_vprintf->
monitor_hmp_vprintf, and monitor_printc->monitor_hmp_printc, changing
the first parameter from Monitor * to MonitorHMP * to enforce type
safety. The implementation is also simplified: monitor_hmp_vprintf now
directly calls g_strdup_vprintf + monitor_puts, removing the virtual
dispatch via moncls->vprintf.

The dev_print() callbacks are temporarily using the MONITOR_HMP(mon)
cast, they are fixed in the following commits.

Early return in qemu_vprintf() if "hmp" is NULL, relying on
monitor_hmp_vprintf() handling NULL case is a bit uncommon.

Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20260828-qemu-no-hmp-v5-37-9227de146347@redhat.com>