hw/core/null-machine: Print error message when using the -kernel parameter

hw/core/null-machine: Print error message when using the -kernel parameter

If the user currently tries to use the -kernel parameter, simply nothing
happens, and the user might get confused that there is nothing loaded
to memory, but also no error message has been issued. Since there is no
real generic way to load a kernel on all CPU types (but on some targets,
the generic loader can be used instead), issue an appropriate error
message here now to avoid the possible confusion.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1488271971-12624-1-git-send-email-thuth@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>