coverity warning about possible missing error check in v9fs_request()

[Peter Maydell]

Hi; Coverity has just come up with a new warning (CID 1438968) about
an unchecked error return value in the 9pfs code. (I'm not sure why
now -- the code in question is unchanged since 2011; probably some
other callsites changed enough to trigger the "other callsites check
return value" heuristic.)

Anyway, in the middle of v9fs_request() is this code:

    /* marshal the header details */
    proxy_marshal(iovec, 0, "dd", header.type, header.size);
    header.size += PROXY_HDR_SZ;

    retval = qemu_write_full(proxy->sockfd, iovec->iov_base, header.size);
    if (retval != header.size) {
        goto close_error;
    }

This is apparently the only call to proxy_marshal() that does not
check its return value -- is it missing a check?

thanks
-- PMM