vl: Print error when using incorrect backend for debugcon

When using an incorrect backend for the debugcon, QEMU exits silently
without any error indication, which is confusing.
Add a message that the character backend is invalid.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20181011171254.32428-1-philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
1 file changed