Skanky READ_WRITE_EXACTLY fix

    364 #define READ_WRITE_EXACTLY(rw, zero_is_eof, constdata)                    \
    365                                                                           \
    366   int libxl_##rw##_exactly(libxl_ctx *ctx, int fd,                        \
    367                            constdata void *data, ssize_t sz,              \
    368                            const char *source, const char *what) {        \
    369       GC_INIT(ctx);                                                       \
    370       ssize_t got;                                                        \
    371       int ret = 0;                                                            \
    372                                                                           \
    373       while (sz > 0) {                                                    \
    374           got = rw(fd, data, sz);                                         \
    375           if (got == -1) {                                                \
    376               if (errno == EINTR) continue;                               \
    377               if (!ctx)                                                   \
    378               {                                                           \
    379                   ret = errno;                                            \
    380                   goto out;                                               \
    381               }                                                           \
    382               LOGE(ERROR, "failed to " #rw " %s%s%s",                     \
    383                            what?what:"", what?" from ":"", source);       \
    384               ret = errno;                                                \
    385               goto out;                                                   \
    386           }                                                               \
    387           if (got == 0) {                                                 \
    388               if (!ctx)                                                   \
    389               {                                                           \
    390                   ret = EPROTO;                                           \
    391                   goto out;                                               \
    392               }                                                           \
    393               LOG(ERROR,                                                  \
    394                      zero_is_eof                                          \
    395                      ? "file/stream truncated reading %s%s%s"             \
    396                      : "file/stream write returned 0! writing %s%s%s",    \
    397                      what?what:"", what?" from ":"", source);             \
    398               ret = EPROTO;                                               \
    399               goto out;                                                   \
    400           }                                                               \
    401           sz -= got;                                                      \
    402           data = (char*)data + got;                                       \
    403       }                                                                   \
    404       return 0;                                                           \
    405                                                                           \
    406   out:                                                                    \
    407       GC_FREE;                                                            \
    408       return ret;                                                         \
    409   }