posix-aio-compat: Fix error check

Checking for nbytes < 0 is pointless as long as it's a size_t. If we want to
use negative numbers for error codes, we should use signed types.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

posix-aio-compat.c

diff --git a/posix-aio-compat.c b/posix-aio-compat.c
index ac247e1..dc14f53 100644
--- a/posix-aio-compat.c
+++ b/posix-aio-compat.c
@@ -117,7 +117,7 @@
     if (ret) die2(ret, "pthread_create");
 }
 
-static size_t handle_aiocb_ioctl(struct qemu_paiocb *aiocb)
+static ssize_t handle_aiocb_ioctl(struct qemu_paiocb *aiocb)
 {
 	int ret;
 
@@ -136,7 +136,7 @@
 	return aiocb->aio_nbytes;
 }
 
-static size_t handle_aiocb_flush(struct qemu_paiocb *aiocb)
+static ssize_t handle_aiocb_flush(struct qemu_paiocb *aiocb)
 {
     int ret;
 
@@ -176,7 +176,7 @@
 
 #endif
 
-static size_t handle_aiocb_rw_vector(struct qemu_paiocb *aiocb)
+static ssize_t handle_aiocb_rw_vector(struct qemu_paiocb *aiocb)
 {
     size_t offset = 0;
     ssize_t len;
@@ -199,10 +199,10 @@
     return len;
 }
 
-static size_t handle_aiocb_rw_linear(struct qemu_paiocb *aiocb, char *buf)
+static ssize_t handle_aiocb_rw_linear(struct qemu_paiocb *aiocb, char *buf)
 {
-    size_t offset = 0;
-    size_t len;
+    ssize_t offset = 0;
+    ssize_t len;
 
     while (offset < aiocb->aio_nbytes) {
          if (aiocb->aio_type & QEMU_AIO_WRITE)
@@ -230,9 +230,9 @@
     return offset;
 }
 
-static size_t handle_aiocb_rw(struct qemu_paiocb *aiocb)
+static ssize_t handle_aiocb_rw(struct qemu_paiocb *aiocb)
 {
-    size_t nbytes;
+    ssize_t nbytes;
     char *buf;
 
     if (!(aiocb->aio_type & QEMU_AIO_MISALIGNED)) {
@@ -308,7 +308,7 @@
 
     while (1) {
         struct qemu_paiocb *aiocb;
-        size_t ret = 0;
+        ssize_t ret = 0;
         qemu_timeval tv;
         struct timespec ts;