qemu: avoid debug prints if debugcon is not enabled

qemu: avoid debug prints if debugcon is not enabled

In order to speed up the boot phase, we can check the QEMU
debugcon device, and disable the writes if it is not recognized.

This patch allow us to save around 10 msec (time measured
between SeaBIOS entry point and "linuxboot" entry point)
when CONFIG_DEBUG_LEVEL=1 and debugcon is not enabled.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>