bhyve fopen failure

I'm porting some code to bhyve and am getting a failure I don't
understand. This is git as of af11c2029006 FWIW.

The code in question is for an emulated device and looks like:
    dbg = fopen("/tmp/bhyve_ata.log", "w+");
    if (dbg == NULL)
        perror("fopen");

Running this fails with:
    fopen: Not permitted in capability mode
Googling suggests this might be capsicum related. If so, what do I
need to change to allow writes to a debug file?

--chuck