monitor: Simplify how -device/device_add print help

Commit a95db58f added monitor_vfprintf() as an error_printf()
generalized from stderr to arbitrary streams, then used it wrapped in
helper out_printf() to print -device/device_add help to stdout.  Use
qemu_printf() instead, and delete monitor_vfprintf() and out_printf().

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417190641.26814-16-armbru@redhat.com>