coroutine-io: Return -errno in case of error

In case qemu_co_sendv_recvv() fails without any data read, there is no
reason not to return the perfectly fine error number retrieved from
socket_error().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-16-git-send-email-mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-coroutine-io.c

@@ -45,7 +45,7 @@ qemu_co_sendv_recvv(int sockfd, struct iovec *iov, unsigned iov_cnt,
             if (err == EAGAIN || err == EWOULDBLOCK) {
                 qemu_coroutine_yield();
             } else if (done == 0) {
-                return -1;
+                return -err;
             } else {
                 break;
             }