Perhaps s/virtio/VirtIO/? And s/passtrough/passthrough/.

This should be with the other sys/* headers. In general we also try to sort headers within their respective groups (though sys/param.h should be first for the kernel headers as it is now)

I think you don't need to strdup rootpath. The existing bytes in 'opts' will be left alone since the earlier strsep() will have skipped over it already.

This means you can't put 'ro' first. I think it would be more flexible to think about handling syntax errors, etc. Also, if read-only mounts aren't supported, should we instead fail with an error until such time as it is supported?

while (...)  {
    if (strcmp(opt, "ro") == 0) {
        printf("virtio-9p: read-only mounts not supported\n");
        return (-1);
    } else if (strchr('=', opt) != NULL) {
        if (sharename != NULL) {
            printf("virtio-9p: more than one share name given\n");
            return (-1);
        }
        sharename = strep(&opt, "=");
        rootpath = opt;
    } else
        printf("virtio-9p: unknown option '%s'\n", opt);
}

Maybe use a continue and then still check for invalid options:

if (strcmp(opt, "ro") == 0) {
    DPRINTF(...);
    ro = true;
    continue;
}

printf("virtio-9p: unknown option '%s'\n", opt);

Maybe defer allocating the softc to avoid leaking the memory here (though we exit immediately, but recently folks have been adding free()'s to appease coverity)

This set of rights aren't used?

It's fine to use VT9P_MAXTAGZ directly rather than strlen().

This doesn't check for overflow. For example, what if the sharename is longer than VT9P_MAXTAGSZ? I would also probably just strdup() the tag name instead of padding the calloc and using strncpy, or else use memcpy or some such if the tag is not supposed to be NULL-terminated. In addition, it might be nice to defer allocating the softc until here to avoid having to free() it in earlier error conditions to avoid leaking memory:

if (strlen(sharename) > VT9P_MAXTAGSZ) {
    printf("virtio-9p: share name '%s' too long\n", sharename);
    return (-1);
}
sc = calloc(1, sizeof(*sc));
sc->vsc_config = malloc(sizeof(struct pci_vt9p_config) + strlen(sharename));
sc->vsc_config->tag_len = strlen(sharename);
memcpy(sc->vsc_config->tag, sharenane, sc-vsc_config->tag_len);