The CAN devices can currently be used to crash QEMU, e.g.:

$ x86_64-softmmu/qemu-system-x86_64 -device kvaser_pci
Segmentation fault (core dumped)

So we've got to add a proper check here that the corresponding
bus is available.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <1521193892-15552-2-git-send-email-thuth@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>