Introduce support for
<serial type='pty'>
<target type='isa-debug'>
<model type='isa-debugcon'/>
</target>
<address type='isa' iobase='0x402'/>
</console>
which is used as a way to receive debug messages from the
firmware on x86 platforms.
Note that the default port is hypervisor specific, with QEMU
currently using 0xe9 since that's the original Bochs debug port.
For use with SeaBIOS/OVMF, the iobase port needs to be explicitly
set to 0x402.
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>