pc: disable the BOCHS BIOS panic port

We have an OS which writes to port 0x400 when probing for special hardware.
This causes an exit of the VM. With SeaBIOS this port isn't used anyway.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-By: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
