qemu: check for QEMU_CAPS_DEVICE_VHOST_VSOCK
    
    My commit b8b42ca added support for formatting the vsock
    command line without actually checking if it's supported.
    
    Add it to the per-device validation function.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1291851
    
    Reported-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Ján Tomko <jtomko@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>